Yeah I went to Darklight Tower to recruit Illia, but when I started the Repentence quest, she wouldn't budge from the spot I found her at.

So I went up ahead and killed her mother myself and completed the quest.

I recruited her as a follower and she still WON'T MOVE. I tell her to wait here and follow me again, and she still doesn't move an inch.

What the heck is wrong with this? It's like she's glued to the ground

have you tried using fus ro dah on her?

Reloading from an earlier save often fixes these sorts of problems.