The last pic looks like infrequent watering.

Topping off water in the reservoir once a day

Waay to much watering. Peppers like moist soil and they actually like the soil to kind of dry out between waterings. If you over water peppers(most common issue people experience), you'll get folding, rotted peppers, etc. Roots need to breath, and if they cant, they will start drowning and die. Dead roots = dead plants

I grow a TON of peppers in 12" pots. Here's some tips

1. Don't over water, let the soil dry out. Get a watering gauge. Peppers will tolerate dry soil much more than wet soil. Pick your pot up when its real dry, water it then pick it up again and you'll feel its much heavier, almost 2x heaver. Remember that feeling and use that to gauge when to water. Or stick your finger in an inch or 2, if it's bone dry, water them.

2. Use potting soil. If you use top soil or garden soil you run the risk of the roots getting compacted, which wont allow the roots to move and breath. In one of your pics it looks like you have some perlite which is good but i dont see it in the others.

3. Dont wet the leaves, dont water the pepper from above the plant, water at the soil level. Wet leaves will breed fungus/bacteria on the leaves. Not only that, when the sun is out, water on the leaves act as a magnifying lens and will burn the leaves.

4. Your leaves are freakin huge, ease off the nitrogen. Nitrogen will produce huge leaves. Phosphorus will produce more flowers, potassium will help with overall health. Use a mild fert once a month or so, something like 5-5-5 or less.

In your second pic, looks like you might have used some fert that was too high and it killed the new leaf growth. Dont use fert for awhile and just water them every other day or every 2 days to wash out the bad fertilizer. Peppers are resilient and usually spring back.