I've noticed that the mutations at higher tiers do not seem to occur until AFTER I get one with a single mutated leaf. I've been stuck at a tier for nearly an hour, no mutation. As soon as I buy a mutant petal and breed it, all the other mutants appear within, say, two minutes. I didn't time it, but I was sitting there clicking, so it couldn't have been very long. I believe there was one tier where I finally did get a random mutant after a long wait, and the same rapid following mutations occured. And when breeding two identical clovers together, mutations occur every so often. So is it getting "stuck" before that first mutant occurs?
Currently on tier 24. Breezed through tiers 20-23 by buying the first mutant segment from the alter, but until I get more gold for the next mutation I can only assume I'm stuck.