Only you know how much fertilizer you have been giving them. If you think that you haven't given them enough then that's probably the problem. It's true that in flowering you want to give more P and K and less N. This will cause leaf yellowing near the end of flowering. However, you say you are about halfway and that is too early for yellow leafs unless it's only the bottom ones.

Don't believe anyone who says you don't have to fertilize in flowering. Only if you use a properly amended organic soil can you get away without fertilizing for any length of time.