You will have to correct fotos to get true colors. It's hard to diagnose purple plants.

Yea ill take some new pics, hang on :)

My Chinense species plants, my reaper especially, are more sensitive to nitrogen as well. I had a very similar problem and had to lower the amount of nutrients below that of what my annuums were setting fruit through before my reaper followed suit.

Okay thanks guys, i have tried using 5ml/l of chili focus now, gonna fertilize them once or maybe twice a week and see if it helps. I'll keep you updated! Some more info for you: Like a month ago these plants were in 2 gallon containers and i had a huge aphid problem in my tent, which caused the flowers to drop. I got rid of the problem with pyrethrum 5ec in a couple of days and there are 0 aphids in there now. Then i transplanted the plants to 7 gallon bucket, and they all started growing really well. The soil is regular potting soil mixed with a bit of compost and perlite. Maybe the compost contained too much nitrogen? The most weird thing is that all of my plants has the same soil and growing conditioning, and only one is producing peppers

+1 to plants look fine...but...media looks saturated, last pic above. Roots need air along with water to uptake nutrients.

Well i looked up what soil mix would be good for peppers, but how would you suggest getting more O2 to roots?

Compost doesn't have a lot of nutrients left by the time it is done cooking, and in pots you don't have the benefits of an active soil web to help digest and convert to forms available for plant uptake. So I would not count on much, if any, N coming from the compost. Not that peppers need a lot. Also be aware that aphids can be attracted to high N levels in plants, which one reason why they hit the new growth harder and sooner. You might check your pH since that can affect Ca availability, as can watering too frequently. I am seeing some edema on some leaves, which could indicate over watering.

I don't think you have a N deficiency; the plants look nicely green. Sometimes high levels of N can keep the plants in a foliar growth phase at the expense of setting fruit. Generally peppers require less N than ,say, tomatoes. I've noticed from years of growing outside that peppers can take a long time to initially start setting fruit, especially the chinense types. Cayenne were almost always one of the earliest types to start setting, so it isn't always useful to compare milestones between varieties.

Also note that blossom drop is a physiological reaction to one or more of multiple causes. You can search the term on this forum. Some of the causes are more manageable than others. Sometimes you just have to wait it out. At least growing indoors you don't have the time pressure of trying to get a harvest before the first frost :)

Flower drop probable causes:

1. Day temp too high >95F

2. Night temp too low <65F or too high >85F

3. Too much nitrogen fertilizer

4. Too much water

5. Low light levels (reduces fertility).

6. Very low humidity (reduces fertility)

7. Poor air circulation (air circulation contributes to
pollination).

8. Lack of pollinating insects.

9. Size of pot

10. Too much mineral in feedwater.

11. Too much grower attention/anxiety.