Do you have them in ample sunshine? Dalhias do like water but too much will cause this condition. They need at least 4 hours direct sunlight for the dwarf variety.

To much water will cause this problem. make sure the water is only applied to the soil and not on the leaves and buds themselves. Deadhead all the buds and and let the soil dry out and they should rebloom for you. I have dahlia's in large pots and I only water when the soil is dry below 3 inches or when the dahlia's themselves start to wilt. Good luck!

Willow