Actually I just realise Aussie cannot get Illia for some strange reason, even though she did the quest at Darklight Tower. I will try it again, in case it was Braggen with that problem.
Have you already had Illia as a follower on that character before? If so, then this is the problem. If not, then I'm unsure as to what the problem might be.
Illia is severely bugged in that, once recruited and dismissed, she can
never be acquired as a follower again because the dialogue option for "Follow me, I need your help." simply drops away. The bug activates once you load/reload a game after initially recruiting her (which is inevitable at some point). So, in my case, I recruited her as a follower and never dismissed her. Fought with her through many levels (she's actually an excellent follower too, which makes the bug all the more unfortunate) and when I decided it was time to make her my steward, I did so.
Now, because of the aforementioned bug, I can never use her as a follower again, but she's my steward now at least, which is ultimately what I wanted.