Why can I not get a girlfriend?

I feel like I am doing something very wrong. At one point in my life I would break up with one girl and maybe 2 months later have a new girlfriend... that went on 3 or 4 times and now I am stuck in a never ending single life. I will say I am very shy, but I am extremely outgoing if I am with friends (which I am 99% of the time). One of my major problems is (and this happens to me constantly with almost every girl I know) is that I get flaked on non-stop. We make plans and when the time comes they ignore my calls and texts and the next time I see them they act like nothing is wrong. I just don't get what that is all about. I'm not conceited, but I am not an ugly guy, not overweight, never smell bad... I am always well groomed and take showers as well as brush my teeth daily. I mean, I guess I sort of have problems meeting girls now (not sure why), but if I do meet them I get flaked on. I get flaked on by girls that have come on to me... how in the hell does that work? I don't even get it. However, there is this one girl that I feel strongly for and she is one of the only girls I know who ALWAYS responds to calls and/or texts. She has a very busy life (family related stuff as well as school and work) so I tend not to pester her with getting together. I let her come to me, which she does over time. I don't know if it's the right time to go after her because she is going through a lot. She also dropped the "he is like my brother" bomb on me, but I feel like there could still be something. But aside from that, I just don't know what I am doing wrong in other cases. It's irritating me so much... Anyways, thanks for reading this to all who do so. Any advice would be appreciated.