My guess is too much fertilizer. If you give them too much nitrogen they will grow more stems and leaves than flowers.
There are special low nitrogen, high phosphorus fertilizers which promote blooming and not stems and leaves. Look on the package of your fertilizer. The first number is nitrogen. The second number is phosphorus. The third number is potassium. So, the second number needs to be significantly higher than the other two numbers, to promote blooming.
