Too much fertilizerYour watering scheduleLack of sun exposureThe soil is too heavy Not cutting your flower often enough
The main reasons your dahlias are not blooming involve the following:
The most important thing to do in order to get your dahlias blooming is: DO NOT FEED DAHLIAS SO MUCH! I do not feed my dahlias anymore after I plant them in the ground.
Dahlias do not require a lot of watering. Water once or twice a week, but make it a deep watering.
Dahlias need sun, at least 6 hours, the more the better. But do not confuse that with heat. If you are in a hot zone, 6 hours may be plenty.
Heavy soil that doesn't drain can prevent blooming. Mix a good quality soil with some compost and apply Osmocote. I mix half of compost (organic fish compost is my favorite) to an organic garden soil.
Don’t be shy, cut those flowers to encourage new growth. If they are not blooming, you may need to cut them more.