I take it you haven't been pinching out flower buds, since you haven't mentioned doing that? Chrysanthemums set their buds according to day/night length, beginning the process as nights become a bit longer than days in summer, rather than on temperature, flowering 6-8 weeks after budset. Perhaps yours went short of water at a critical time, so bud set didn't begin. They're also heavy feeders, so lack of fertiliser might have been an issue, possibly.

I have chrysanthemums that I grow from side shoots on shop bought sprays . They grow tall and this year I only pinched out once which is the reason they are tall and will probably only have one spray of flowers. But there is a tiny bud now on each one now. They usually flower November for me up here in the north.

Mums growing too close to porch lights or streetlights often delay flowering, or, sometimes, stop entirely.