If the plants look healthy and with bright green leaves, it sound like the fruit simply isn't mature enough to have thick walls yet. It seems that no matter what the conditions are, the fruit comes first at the expense of the leaves. Thus I always tell people that if the leaves are a healthy color, don't worry about the size of the plant. Maybe it also takes a lot of years of experience but I can stand 10' from a green pepper plant and tell if the fruit is ready to eat. It just has a certain "shine" to it. You can also use a gentle squeeze. If the walls have any give to them, the fruit isn't ready to pick. You can also "plink" them with your finger. Should sound hollow.
Martin