Ah, crud!
After re-reading this post, I don't think I made my point. It sounds like I'm bragging or something. I'm not!
Many of my semps in plastic pots simply were not draining right. My mix was:
4 parts potting mix,
2 parts perlite and
1 part grit.
I even tried adding more perlite and some course sand. No good.
After noticing that the semps in clay pots were doing just fine, I thought that switching to the clay pots was the best course of action. Being a gardner for many decades, Bill thought we must act right away before we loose any plants.
I should have acted sooner.
I lost Canada Kate and a small rosette from Ysolde.
All seems well with the clay pots, so far.
Is it possible to add too much perlite or granite grit to the potting mix?
