what do I do in this situation?!? Why would he not just tell me...?

So when I walk by guys that are taking to each other they stop talking and they just stare at me and then I can hear them say "did you see her she was fine or pretty" but they don't come up and talk to me. So does this mean I'm not really that pretty if they don't directly say it to me? I don't dress provocativley so I'm pretty sure that's not the case. And when guys do compliment me they always say stuff along the lines of "I bet you have a lot of guys chasing you" but I don't and I tell them that but they think I'm being modest. I'm not so now I just laugh and say sure. If guys do think I'm pretty and I hear them tell their friends that why won't they come up and talk to me. Do I look unapproachable. People say I look stuck up but that's just a guard I use so people don't think they can just push me over.