Just a couple of thoughts off the top of my head:

If it's a species aster, the blooming may seem rather sparse in comparison to most of the hybrids on the market. You should try to find out what you have.

How far from the house is it planted? Reflected heat from the house itself could be frying the plant. I had problems with an aster not blooming the way it should and then realized it was planted too close to the sidewalk. Heat bouncing off the sidewalk was cooking the poor thing. I've also moved all my asters to a SE location because the SW location was just too hot in the afternoon. They're doing much better.

My asters like regular moisture. Drying out too much can cause fungal problems with the lower leaves.

Does it need division? Asters are generally robust growers benefiting from frequent division.

Kevin

aachenelf, it is right next to the house. It's actually surrounded by concrete. The foundatino is behind it, and immediately to the left is the concrete driveway. 2 feet in front of it is sidewalk.

Thank you! I may try moving it and see if that helps. It's not huge enough to divide yet.