If you were "pretty good friends," don't you think you'd know whether she had a boyfriend or not?
She knows, beyond a doubt, that you want to be more than friends because you were chasing her before. I can guarantee you she isn't under the impression that your previous strong interest has gone away and the you now want to be just friends. You don't need to worry about your getting the point to her; you've already communicated more than enough. If you're going to do anything more than give up, you have to put the ball in her court to make her clarify her feelings toward you.
Girls love[i/i] leaving you in ambiguous, anomalous situations (both when you first meet, and when they're dumping you) -- it maximizes their ability to keep you hanging and available for whatever purpose they may have for you as long as they like, to rewrite history, and to put the blame squarely on you when you "ruin the friendship" (or, in some cases, "don't show enough interest in me," if they've gotten huffy that you were too respectful and not supplicating enough). This is not accidental (though it may be inbred). For all that women talk about communication, they are deadly foes of being made to make clear and accurate statements and live with the consequences.
So . . . your options:
1. Decide you really do just want to be friends (and that her "friendship" for you amounts to something more than using you as a member of her entourage or an ego boost because she knows you still want her). Then, you treat her like any male friend, no better, no worse, and (unless you're a bit funny), if you're treating her like your guy friends, you won't be obsessed with romantic thoughts.
2. Decide you want to go out with her (past snafus notwithstanding) and force the issue. How to force it? That's a tough question; women really do have an advantage here because too forceful an approach seems desperate or rude, too wimpy an approach ("let's hang out and eventually she'll get the message I like her and then over a five year plan she will become my girlfriend.") rarely, rarely evokes anything but awkward contempt or pity (no, she won't date you out of pity -- that's another lie, that women are compassionate and "give too much" of themselves.).
I do know what doesn't work: Asking her to "be your girlfriend." Asking her if she "could ever be interested in you as more than a friend." Telling her you "have really strong feelings for her."
You need to change the ground rules and let her know that (a) floating along as ambiguous hang-out buddies; and (b) being platonic friends is not what you want. All I can say is force the action -- dozens of guys here would say something along the lines of "just kiss her," or "just ask her out on a "date," explicitly" or "just tell her you're interested in her as a woman, not as an androgynous friend," and I can't think of a much better approach. Note that I am not saying this will definitely work in the sense of making her your gf; there's a chance (maybe a strong one) that it will evoke just more LJBF (to which the stock response, again probably the only one, is "I sought you out as a girl, not as just a friend" -- implying that it's either/or. But this approach will put an end to the ambiguity she's been purposely cultivating/allowing.
3. Forcing the issue isn't risk free. Yes, if she isn't interested, she'll make you look/feel like the bad guy for "ruining" the friendship. But if you've already decided that mere "friendship" is not enough for you, what's the harm of a little female spin-doctoring to make you feel like the clumsy oaf? Worse happens every day. You could also skip directly to this mentality and kind of allow contact with her to fade away without even forcing the issue, if you suspect as I do that she will definitely blow you off.
I've had, I think, one instance of turning a friend into a gf, and it wasn't a case when I had already been blown off by her after showing interest, and then came back and was successful the second time (rather, we met and became voluntarily-platonic friends when we both had other stuff going on, and then later figured out that we liked each other). I've had a lot of other times when I'd have like to transform (involuntarily)-platonic relationships to more, and it just doesn't seem to happen often.