I've tried for 5 years to grow red bell peppers with very little success. If I let the green peppers go longer, they either get too large for me and, usually, those which turn red already have some sort of disease or rot. I would like to grow ones like they sell in the grocery store.
Growing Red Bell Peppers
Certified GKH Gardening Expert
Perhaps a different variety of red bell pepper (or other sweet pepper) would suit your garden better. If you are in a colder region than the variety's ideal, it will have trouble turning red before the season turns cooler (or diseases get to it).
Look for varieties that have a shorter days-to-harvest rating or that are bred for cooler climates. You may also have better luck with a mild variety of pimento pepper.