I'm not sure what you're asking. By not growing, do you mean the tops aren't getting any bigger? That would not be uncommon, but you're looking for blooms. Anytime after the potatos bloom, one can harvest some of the new potatos, but for larger potatos, you'll want to wait until after the top, green growth actually yellows and dies back.
Even then, the tubers are continuing to grow and will do so until freeze-up... the longer you wait the more and bigger they'll be...