First of all thank you for the detailed pictures. Even with the detail that can be observed here we may not be able to come up with a definitive answer. This is what I notice:
- from the first picture the plants appear to have been started from cuttings as is the usual practice but the stems of the cuttings look a little short. Depending on the time taken to grow them on they may not have had a well developed root system when you got them. This means the roots would have been close to the surface and would not have taken well to a normal "water thoroughly and let dry" cycle as the roots might not have grown down to the soil area lower down in the pot.
- the roots I see are a mixture of healthy (firm and white in the bottom picture) and less healthy (brown and papery in the middle picture)
- the soil moisture level seems to be moister at the top and bone dry at the bottom. This is the opposite of what a thorough watering should yield. You would expect the bottom of the root ball to be moist and the top to dry out first.
Your next question would be what's going on?
I think the watering regime is off. Possibilities that you can test yourself are:
- instead of enough water to wet the entire root ball and then let the top third dry out it may be that only enough water to wet the top of the root ball is being applied. A sequence of: a little bit of water, roots grow at top where water is, soil dries out, plant wilts more than it should if the roots were vigorous throughout the entire root ball could be happening. You would expect to see better root growth at the top, less new healthy roots in the bottom part of the root ball.
- or it could be too much water causing stress. If there was no drainage and so much water was applied that the base of the root ball was awash then you would see dead roots in the bottom and weak roots at the top and, likely, you could see opportunistic fungal/viral spotting on the leaves as they attack a weakened plant.
I still think a wicking system will solve most of the issues. With a capillary wick and putting the plant in a pot it can draw up as much water as it needs but not too much. You can also see if the plant is sitting in water or not.
It would be pretty easy to do this for one trial plant. Other tests would be to take some cuttings from the healthiest plants, root them in water and then pot them up. If, by some chance, your plants have a fungus/virus/bacteria they will continue to spot on the leaves even as cuttings.
Inevitably this is speculation so I encourage you to experiment. Try other easy tropical such as a trailing small leaved philodendron or a Sansevieria compacta Hahnia for a drier area