THe problem with these women is that they're selfish. They want to be nice to everyone so that no one hates them. Girls, after all, care much more about how people think of them than guys, thus being nice to everyone means everyone will be nice with them. Her issue, however, is that most men take a girl being nice to them as them hitting on you. You guys make the assumption that they have an interest in you, only to end up being hurt due to the fact she has no interest it seems. In reality, they're probably like most women and are looking for "mr. right", and don't see you as such.
I had a similar problem, I couldn't tell if this girl I was trying to date was just being nice to me or everyone else. Long story short, she was nice cause she wanted me, and now we're engaged. My advice is don't give up, she may be unsure if you're worthy of her but its your job to make her convinced you're good. THe biggest message: both sides make assumptions, don't make any and you have a better chance.