The het mating game (I'm told gay folks play their own, much harder, version) is basically governed by the iron law that you resemble your competitors. Ergo, if you see a lady and instantly want to ask her out, she probably gets that a lot. If she does, then she's probably said yes already if she's the sort of person who would do so.

That isn't to say you shouldn't ask out the girl you are most attracted to. Sometimes you are the first guy, after all someone has to be. Rather, you should ask out people you aren't immediately attracted to. Hotness -> dating works more or less like bachelor degree -> career. Its only important right at the start. If you like her it won't matter that she didn't instantly ping as possible match.