Could you please tell me why my chilli pepper plant leaves are curling. The plant has grown to about 15" tall and has produced some chilli peppers. The peppers are already maturing but new flowers are falling off and the leaves are curling. I don't think the sun is too much because it's in the direct sun for only a few hours; it doesn't have any pests etc. on the leaves; it doesn't seem to be overwatering as the soil is just moist. What am I missing? I have checked a lot of sites but I can't find anything concrete.
Curling as in cupped and deformed? Or curling as in wilting?
Calcium is needed for the construction of cell walls. If calcium is missing when cell construction occurs, you get something that looks like this:
That's a fairly extreme example, but illustrates the cupping and curling of the leaves.
Soil is likely to be deficient in calcium in high rainfall areas and where soil is acid.
If you have reason to believe your soil is low in calcium, your options for replenishment are limited. Calcium compounds are rarely soluble in water, such as lime (CaCO3). Lime simply wouldn't break down in time to do the plant much good, plus it would increase soil ph. Water soluble fertilizers (miracle-gro, etc) usually don't contain calcium, since its so hard to make it dissolve and stay dissolved without producing salts. The best option would be calcium nitrate. Its very soluble in water.
In my experience with calcium deficiency in plants, the plant continues to grow and produce just fine (except for deformed leaves). Tomatoes will have "blossom end rot". I believe melons will too. Maybe an increased susceptibility to disease will be noticed, but the plant doesn't die or stop producing. So if flowers are dropping, maybe calcium isn't the only mineral lacking. If that were the case, then something like miracle-gro and calcium nitrate would be the treatment.
That should save this year's crop of peppers, but next year you should start with calcium rich soil. It would be wise to research soil calcium more, since its such an important part of human health.
Based on your comment in Bamboo's answer you said the leaves were wilting and shrinking. Too much chlorine can cause wilting, and it can kill beneficial microbes that might help the leaf size; I know from experience that microbes seem to help leaf size in at least in cucurbits.
You might try filtering the water on this plant, if you're using city water.
Curling can be caused by too much humidity/fungi, too.
With this and Bamboo's words about calcium in mind, some potential things to help may include worm castings (for microbes), potassium sulfate (not chloride), and basalt rockdust.
Rockdust contains calcium and silica which go hand in hand with potassium to strengthen plants. Calcium without sufficient potassium and potassium without sufficient calcium can both cause issues.
There are probably other things to help with leaf size, but I need more experience to tell you about that. I would experiment with fertilizer, and such, myself.