Perhaps you are giving it too much fertilizer. That can cause a lot of plant growth without the flowers.

...Or else fertilizer with too much Nitrogen, The first of the NPK numbers. Some brilliant person on this forum suggested tomato food spikes. Push in two per plant and they are slow release (redo every 2 months) and those dahlias will bud like a hot d**n. Worked for me! When you think of it, they are just the perfect formulation for dahilas. With tomatoes you want lots of blossoms which would ultimately result in lots of fruit. With dahlias you want lots of blossoms too. So simple, and it works! It really does!