Someone blocking her magic was the only thing he could think of. Well, there was one other idea, and that was her sort of blocking herself. It might explain why she wasn't feeling anything, as far as a block would go. Still, he figured that she had gotten past the bigger worries of how she was going to turn evil again or anything like that. Hopefully, if she came close again, they could all realize it before it was too late and pull her back before she was set lose.
Another thought popped into his head. "You felt the spell leave like normal?" he asked. He knew what she was talking about. He had performed enough magic to know the feeling when it feels like a spell was complete. "Do you think that maybe it worked on someone else?" Just an idea, really. It would explain why it felt like it worked but then didn't work on Angel/Angelus.
"It might have something to do with how he lost it. Some particular spells that steal souls can have certain effects when you try to break them." And they usually had ways of protecting themselves, or the warlock/witch in question would find a way to make sure that the witch trying to break it wouldn't survive trying to break the spell.
"Tell me something else. Have you tried to perform any magic since then? Has it not worked?" If she had tried and failed, that would lead to more of a general block. If it had worked otherwise, then it probably had more to do with possibly how Angel lost his soul in the first place than Willow herself, which might help relieve her fears a little as well.