Wish I knew. Some of mine and my neighbours have done that as well. His are just beginning to get buds now. Mine have been consigned to the compost heap.
Read this
https://www.gardenerreport.com/cosmos-not-flowering/

Very interesting reading about cosmos not flowering. Think I might nit bother next year as most of mine haven’t flowered this year. Think my soil is too rich.

well this year's weather hasn't helped with such fluctuations. As the article says the light, nutrient levels and amount of water will all impact on how any plant grows.

It could also be the variety,as I have the shorter ones,and they have been wonderful ,and still are,but my Neighbour has the tall ones,which are all white,and only started to flower two weeks ago. My soil isn't rich where they are planted either,but in full sun,so that could be another factor.I wouldn't give up on them yet,Charnwood .:o) .

Some of my cosmos are very tall and started to flower three weeks ago .Plenty of buds but weather forecast for week after next is very cold according to met office. I'd hold on for a while, which is what I'm doing; the flowers are very welcome albeit rather late.