Maybe you didn't rest them enough? Mine in the greenhouse are still flowering and still tight Winter rosettes. They are just now starting to come out of it. Or perhaps as you mention the majority are simply too small/young.
Mark I am by no means a Ping. expert either. My understanding is that most of the species flower from the Winter rosette stage. Flowering can take place Winter through Spring from my experience, either directly from the Winter rosette or as the plant resumes normal growth in the Spring. I do however have an unidentified P. moranensis hybrid that flowers continuously all Spring, Summer and Fall and stops flowering during the Winter.