I sort of have the same problem. I'm not really shy, but I've never really been able to flirt with any girls, and it's becoming a serious handicap. For me, it's not about invading boundaries, although I'd rather not come off as a creeper, it seems like I always get such a tepid response whenever I talk to a new girl, like she'll exchange pleasantries, but past that has no interest in me being anywhere near her. I understand it's partially psychological on my part, but it's become so commonplace that, since all girls I talk to have such a disinterested, insincere introductions, it's become damn hard to even consider them as someone I would want to go out with in the first place. It's really becoming a problem.
I understand the advice of simply practicing flirting in a no-consequences situation where you don't even care if they bite, but even that doesn't feel like it would fix the problem.