I love growing dahlias but they can seem very temperamental at times. Odd shaped blooms or lack of blooms can be caused by many things with weather related issues often being right at the top. Heat can be a huge factor as can the lack of rain. Insects can cause unseen problems. Last year I had one bed that was my worst producer ever. I wouldn't have known it was infested with thrips and aphids if I hadn't put out some yellow sticky strips among the plants. In a short period of time the strips were literally covered with bugs. These insects can sap the life out of a plant.
You might also want to have a soil test done. Sometime certain minerals can become depleted from the soil. Some can be trace minerals that we don't often consider. Tests are worth the cost if done by a quality lab.
This time of year, I always get some odd shaped blooms. I think most of that is caused by the sudden change to cooler fall temps and less sunlight. Many blooms are also smaller now.
