I wouldn't worry too much as long as they look healthy. In my experience, most pepper plants have a very long life in the garden and grow continually [if perhaps slower] as they get older, and there is still quite a lot of growing season left, even more so in a greenhouse. For reference, I planted my peppers [outside] at about the same time you originally posted, and mine are all about the same height as yours, around 9 inches or a little less, and are only just now beginning to bloom. I think a lull in growth at or about this time in their life cycle is usual. It's likely they are redirecting energy from plant growth to preparing blooms and fruit set.