Dahlias always do horribly where I live. They only really bloom for us in mid-late spring and early fall. I imagine it's like what
@BigBill was describing. The hot nights here do almost everything I plant, dirty. Tomatoes stop producing, peppers struggle, impatiens take a break, many annuals just stop growing, and dahlias are sadly a part of that list, too. Interestingly, I know of places that have 100+ degree days for weeks, but have cool nights. Their dahlias bloom still. Hot nights are just a plant's worst enemy.