What kind have you grown? Are you thinking only of bell peppers? There are many different kinds of sweet peppers that aren't bells. Italian peppers I've grown with thick walls include giant Marconi, gypsy, carmen and corno di toro.

The "secret" is to not get in a big hurry to plant them. The soil must be thoroughly warm or you will stunt the plants and they will not do well. Some years, I wait until the fourth of July to plant and those late summer plants in our horrid summer heat do incredibly well.
The other "secret" is that these plants are heavy feeders. In my nitrogen poor soil that means plenty of Blood Meal or Manure, whichever I happen to have on hand.
I have had very good results with "Big Bertha", which is a bell. They ripen to red and they are thick walled and juicy.

Yes, I am asking about bell peppers.
Green, red and yellow.
I tried all 3 last summer,
All had paper this walls.
Those I buy in the stores are always thick walled.
And of course, no variety names are given.

Thank you for responding.
I will try again this year.

Maybe.

Rusty

I planted my first garden this year in Rockwall Texas. We are a little east of Dallas, and unfortunately my bell peppers are already planted in a container. I think I planted to early. Three of the plants have leaves that are turning yellow and curling. Is there anything I can do to help them along?

A lot of it depends on the seed variety. Some want a more tender, sweet and thinner walled pepper and some want a thick pepper. The colored bells at the store are really unripe and then gassed so the walls stay thick and uniformly colored. Odessa Market is a terrific thick walled sweet pepper.
A soil test will help determine problems, and plant peppers close together, use plastic mulch and don't water too much when fruiting.

Julie, I would try feeding it a liquid fertilizer. Schultz Instant Plant Food, which is typically sold for houseplants is excellent for seedlings. And, since it is in a pot, I would bring it inside if that's practical. If you can put it in a south facing window, it will probably be alright until the weather warms up in two or three weeks or so.

If it doesn't perk up dramatically just buy a new seedling and mark it up to cheap education.

this is our method:

len

Here is a link that might be useful: lens growing sweet 'taters and pineapples

Seed variety...inadequate K at fruiting time...cool weather...

...these can all affect wall thickness.

Thanks Donnabaskets! I have moved them inside and will try the fertilizer. :)