Q.growing bell peppers
My pepper plants seem to be healthy with no sign of problems, but the peppers themselves remain small and crinkly with thin outers. What are they lacking?

This is actually a sign that you may be harvesting them a little too early. The walls on the peppers thicken as they age, so the longer you can leave them on the plant, the thicker the walls will become. You may want to leave them on the plant until they just start to turn to their final color as that is when they will reach maximum wall thickness.
Thin walls on peppers can also also be just genetic. Some peppers, especially chili peppers are bred to have thinner walls.