Certified GKH Gardening Expert
That is odd. Are you cutting it back so it's not so tall? If so, that could inhibit blooming if it is cut beyond early summer. Cutting asters and mums back should be done by the end of June and no later. They benefit from pinching until then, too.
They do benefit from full sun. If it only gets west sun, that is just half a day. Here is more: