There, now that THAT'S out of the way...
This may sound judgmental, and I don't mean for it to. I do think there is a certain type of woman that claims to be bi or gay JUST to get attention from another woman... when in reality they aren't really, they're just doing it for attention. Plus, some girls have jumped on the "it's hip to call yourself bi" bandwagon when they really don't belong there. Then, they get to have fun getting attention from another woman, but if it starts to get to be too much they just jet. Obviously not ALL women who identify as bisexual are like that, but I do think there are some that are thinking that way.
Maybe you've been meeting those kinds of girls more lately?
Conversely, it could just be that you haven't met the right girl yet. I had to talk to, get to know, go out with, etc a TON of people who weren't just right for me before I met my current partner. So maybe it's as simple as that too.
Dating is frustrating, and since the majority of the population identifies as heterosexual, the bisexual and homosexual dating pool is automatically that much smaller. So your experiences are likely to be doubly frustrating in that department.
All in all though, it really doesn't sound like it's you. Try to keep your chin up, your positive attitude and respectful attention will be appreciated by the right girl.