The leaf of my green chilli plant keep curling and wither away. I have tried treatment with neem oil pesticide /fungicide but it does not show any improvement
The plant does not flower / produce chillis
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Spider mites are a possibility. They might not respond to an insecticide, as they're not insects (I'm not sure about neem oil). They're pretty hard to eliminate (but the plants should do better after being transplanted outside, if you're going to do that). Reinfection from other plants (including houseplants) in the area is likely, even if they don't show symptoms.
If you have multiple plants per pot, or multiple pots of plants close to each other, that can contribute to foliar problems. I experienced this a lot in 2015 when I was starting my plants inside (to transplant out later, in the spring). Plants (especially indoor ones) enjoy space and ventilation. A fan in the room should also help, if you have packed foliage. Ventilation helps to keep fungus to a minimum. It helps to keep the leaves dry.
If your room is too warm, that can cause foliar problems, too. Plants like warmth, but it's not always the best thing for them (in my experience). If you can keep the soil warm (e.g. with a heat mat) without warming the whole room, you may get the best of both worlds.
If you want lots of peppers, I recommend planting only one plant per pot, giving it a larger pot (with the right nutrients in the soil), and make sure it gets plenty of light. It looks like your window is bright, which is nice.
Variety is also important. For indoor peppers, unless you have sufficient supplemental lighting, you'll probably want to go with plants that can do well in the shade, like Grandpa's Home or Ring of Fire. Some varieties may resist these foliar issues and/or pests more than other varieties, too.
There are other important factors, and I'm not aware of them all, but I'm guessing healthy soil microbes probably play a big role, too.
Adding extra potassium can indeed help to strengthen indoor plants (provided the soil isn't already abundant in it). Phosphorus is supposed to help plants mature faster; phosphorus is also supposed to help with flowering.
For mature plants, you'll want to make sure you're not still using a seed-starting mix, or you'll likely never get much, if any, fruit. Seed-starting mix is depleted of nutrients. I'm not sure what soils are best for encouraging fruit for peppers indoors, but I do know the soil you pick can make a big difference.
In my experience, plants don't respond to nutrients the same way indoors as they do outdoors. So, that's another challenge to getting fruit indoors. There doesn't seem to be a plethora of research regarding getting much fruit on plants indoors, unless you're talking about hydroponic gardening with artificial lighting and such. So, maybe you can help pioneer the research (be it anecdotal or otherwise).
You don't mention the temperatures where you are. I have experienced these situations (and worse) when the temperature drops below freezing. The amount of damage can range from falling leaves to the plant dying if the temperatures go too low.
Another possibility is if you have been growing these plants for a while and never fertilized it. That can cause similar effects.