Raising about a dozen different species of Rebutia cacti from seed. All species gave some germination, and although some were few to germinate, some are still showing after 4 months. The largest though are about 1/4" (6mm) diameter and need separating. The problem:

I.ve been separating the larger ones into 4-5 in a 3" pot. Whether I let them stay in the dry mix for 3-4 days or water sooner, I keep losing 1-2 per pot. Some after a week, some as long as 3 weeks. They just drop over and when examined the roots are totally gone. Suggestions?