I've had a lot of trouble with Jowan myself. I obtained it a few years back, and all of the rosettes that I had bloomed for me. So as soon as I noticed the blooms forming, I divided all of the rosettes and removed the forming flower spikes. Unfortunately all of the divisions eventually re-bloomed or died off with none of them ever producing additional rosettes. The exception was one rosette that earlier this year did finally, after a couple of years of doing nothing, form an extra rosette (or possibly two). However, the last time I saw it, it was blooming again. I'll have to track it down to see what as become of it. Hopefully its finally decided to behave like a plant that wants to survive.
Frequently when closely related plants will not cross, they have different numbers of chromosomes. This does not appear to be the case, but there is an apparent difference in the pollen shape between Jovibarba and Sempervivum, which might be significant.
The following article, although it gets a bit technical at times, does contain some useful information as well as speculations about this issue.
http://stalikez.info/fsm/semp/...
