In my experience, usually lack of nitrogen, though that's more of a problem in containers. Note that too much nitrogen will cause them to grow like nuts, but not bloom. It is better to err on the side of caution, when applying plant food.

Thanks tugbrethil...they are in containers. I fed them with liquid seaweed so that could be the problem.

Could be, there's not much nitrogen in seaweed extract. Good luck!

Thanks!