So I have Umbra, and I summon a dremora lord via scroll. I kill it, while soul trapped, and I get the message "There is no gem in your inventory large enough to capture this soul."
...k.
So I summon a plain ole' Dremora. I get the same message. Wut?
From the wiki:
"..It is essentially a reusable Grand Soul Gem."
So, why can't it capture the soul of a Dremora? And before you ask, yes, it's empty.