Society has taught men that its wrong to check girls out and that if you do you shouldn't let them find out.So you appreciated the attention and searched for eyecontact for them to approach you, what happened is that they took it as "oh no she looked, she noticed I checked her out" and turn away in the hopes you didn't notice.
They are usually the shy guys, they might be intimidated or have no idea how to approach you because your always around friends or not and don't want to intrude even if they where interested.
Give up on those guys and hope you'll meet some guys who have no issues with checking you, holding eye contact and then approaching you to get to know more about you.
"DAMN she's fine...*you make eye contact*...shes too good for me- best look elsewhere" ...maybe that's what's going on :p
Intimidated/shy. You're probably a little "too" well maintained so they figure you're out of their league, automatically stuck-up, a bitch, whatever. If there's a guy you like just walking up to him and saying "hi" is all you need to do.