Ginia makes small adjustments to his signs here and there, but as the minutes tick away with no appreciable effect, it becomes clear it's not his signing that's the problem or anything with him. A test of him verbally casting the spell and her signing the spell both prove to work perfectly.

Which means ultimately:

"Maybe signing a spell requires knowledge of the language behind it after all. I suppose it would be too easy if people could copy the signs for the spells."

And frankly, if it means only a handful of shinki can sign spells well, maybe it's for the best.