Why are my repotted plants dying/not growing?

Question

I have recently re-potted my plants as the soil they came with was either clay or too compact or they were already root-bound or the soil was not able to hold moisture. They have been shedding leaves/not growing/dying, since I repotted them. It has been atleast 1 month before I repotted any of the plants. I did repotting in late evening.

New soil mixture: Out of 10 parts, I used 5 parts garden soil, 3 parts Vermicompost and remaining 2 parts a mixture of river sand, Perlite, Neem Cake Powder (as a pesticide and fungicide) and Coco peat.

I also added Epsom Salt to help them cope with repotting stress.

Almost a week has passed and either their growth is completely stunted or they are shedding leaves.

I keep all the plants in the balcony facing the North direction. They receive good sunlight and warmth during the day for at least 4 hours, but since 5 PM the temperature starts to drop from 24-27 degrees Celsius (at 1 PM) to almost 12-13 degrees in the night. (It's winter here)

I noticed today that the pH level of the soil is minimum 8 (the meter shows only upto a maximum of 8, so it could be more than that also). I checked and the soil is moist but since it has good drainage, it is not overwatered. Putting my fingers deep into the soil, and just a little bit of soil sticks to my fingers, no water sticks on my fingers even if I squeeze the soil in my palm. The soil is also quite porous and the pot has drainage holes.

So, today I added Humic acid (1 teaspoon in 1 Liter of water and evenly distributed almost 150 ml each in all the 6 repotted plants), to increase acidity of the soil and to also increase the absorption of nutrients in the soil. I could have also put vinegar, but my plants are like my children and I want to only give organic and healthy things to them.

The status before repotting:

1. Marigold plant had all it leaves dead before I repotted it. While repotting I noticed that the soil it came with was clay and because of that the soil had become very hard. So, I carefully soaked it in water and very very carefully removed the clay soil and put it in the new mixture.
2. Bougainvillea plant was showing good flowering but 2 days before repotting it had full bloom and all its leaves shed the day before repotting -- all pink and green leaves shed before repotting. The soil was too porous and was not able to hold moisture. (It would start draining within 1 second of watering even a little bit of water -- so the soil quality was very bad -- but since it is drought resistant - it was surviving). Before that, it was growing nicely.
3. Ixora plant had shed most of it's leaves before turning brown and had completely stunted growth (no new leaves) -- I know it's a summer plant and currently the temperature in my area ranges between 13 degrees minimum at night to almost 24-27 degrees maximum during the day. Before repotting, it had two leaves, one had almost completely turned brown and the other was slightly brown.
4. Tuberose plant had a very stunted growth. The soil was of good quality, though a little compact. So, without hurting the root-ball, I moistened the soil around the root-ball and removed the soil a little bit. The plant had become root-bound, so I think that was the cause of the stunted growth.
5. Passion Flower plant was growing nicely. The soil was also good.
6. Poinsettia plant was having many leaves, but it was not growing and all the leaves (red and green both) were drooping. The condition was becoming bad with every passing day.

The status after repotting:

1. Marigold plant is not showing any growth. No green branches and no leaves or flowers. But I am not going to accept that it is dead. The branches feel very brittle to touch, I am scared, but I know that it not going to die yet.
2. Bougainvillea plant is still devoid of any leaves, but the branches are not brittle.
3. Ixora plant still has those two leaves in the same status. I can see the tip of few branches as green.
4. Tuberose plant is growing quite slowly. All the leaves are green, no yellow or brown tips, but the growth is quite slow.
5. Passion Flower plant has shed a couple leaves before turning yellow. It still has almost all of the leaves.
6. Poinsettia plant has shed all of its green leaves. The red leaves are also wilting and slowly dropping, but I noticed that there were some white powdery substance on the red leaves. Today, I saw that the leave which had the most white powder on it is now turned black. It doesn't have any gooey thing on the leaves and no sign of ants too. I think that it would be aphids, so I sprinkled wood ash on all the leaves. Should I remove all the leaves because of it? Is removing all the leaves a good idea?

I really need your help. I love gardening and I want them to grow and take over the whole balcony. But they are not feeling well. What should I do?

Plant pictures TODAY:

ALL THE PLANTS WILL HAVE A LITTLE BIT OF WOOD ASH AND HUMIC ACID ON THE LEAVES THAT I SPRINKLED TODAY.

Soil mixture used:

Marigold:

Bougainvillea:

Ixora:

Tuberose: The slight black color on the leaves is due to the Humic Acid solution.

Passion Flower:

Poinsettia: What you see on the leaves is Hard Wood Ash that I sprinkled today.

Answer 1

So it's what passes for winter right now where you are... First, the African marigold; these plants are only annuals and live for one season, so unless it seeds itself, it will not return. Tuberose is a bulb, and as such, dies back at the end of a summer and needs to rest for a while before restarting growth in spring; it should only be fertilised when it has strong leaf growth present. Poinsettia is usually just a temporary visitor which gets disposed off as it dies off, so that is likely the case with yours, though you can certainly wait to see if it recovers. Your passionflower looks like it might need a larger pot, and cutting right down to the base, where the new leaves are growing. The bougainvillea for some reason, having lost all its leaves, is obviously not growing right now, but hopefully may produce growth a little later on. Ixora requires a moderate to acid soil - it is usually evergreen even in winter, but perhaps it will recover over time - do not feed now, wait till spring, when you can also trim back the tops to encourage growth.

Some general rules; don't feed plants in winter, and when repotting, don't remove old soil unless it falls off as you lift it out of its original pot, just place the plant in a larger pot and fill in around the original rootball with new soil, and water in well. Note also that it's best to pot on most plants in autumn or in spring rather than in winter, which is a time when some degree of dormancy is present. The only time you try to remove soil is if the soil is infested, when the plant can be soaked in water to help gently remove the infested soil - otherwise, root disturbance can at least check any growth, and at worst be lethal. I note you have used some garden soil in your repotting mix - this is not usually advised because it might contain pathogens, but I am aware that where you are, sterilised potting soil may not readily be available. Hopefully the garden soil used will not cause problems. As for why you're not seeing growth on some of the plants, it's only a short time since you repotted, and it's winter - even where you are, that means the days are a bit shorter and temperatures lower.