Mark,
Combo of genotype and environment seem to be involved. Some species like the calcareums and atlanticum don't flower very often. Warm weather seems to increase the % of blooming rosettes. The very warm summer of '17 made virtually every cultivar have a bloomstalk. It was a goldmine for me in crossing all kinds of things together, with 236 different crosses.
Stress also plays a part. I had my first shipment from Fernwood stay in customs for a month. The plants were etiolated and several in that group went right into bloom. Luckily I only lost one cultivar because the smaller rosettes included didn't bloom. I was able to do some crosses with these stressed blooming plants although the seed set was low, at least I had the genes from that cultivar into my seedlings.
Kevin
