Why does she ignore me? Is it a bit of social anxiety; is it because I once did it; does she not notice me...?

OK, this was too long to read. Walk up and talk to her. If you like her, ask her out. If not, don't worry about it. If she is too flakey, and you can't depend on her. And her moods seem to change....I don't know that I'd want to waste my time on someone like that. But, if you like her - talk to her. If not, just get busy on something else.