I just figured it out! Power of the Spheres actually takes two astral gems -- even though the description says it takes one:
This explains why some of my communions lacked a certain oomph earlier in this game. Come to think of it, they lacked a certain ooomph in my previous game, too, where certain spells weren't going off, but I didn't pay it too much attention at the time.
I wish I had known about this earlier. It probably wouldn't have made that much of a difference in this game, although it certainly would have made things much more painful for a certain Lizard Man
