Why would a girl call me to hang out, then blow me off?

Most of the girls I know call me and talk to me like totally randomly. We make plans to chill later that week, then when the day comes they blow me off. I've tried calling them, tried not calling them. Like I tried caring about it and chasing them for a while. When that doesnt work I tried not caring for a while, thinking that maybe when I back off a bit it will draw them in. They obviously just dont care. Im not a freak, Im reasonably good looking, I have goals, I'm nice but not a wuss. I can make a girl laugh, I can hold on an intelligent conversation... Now, all that being said: I'm 25 and the girls I seem to attract are 18, 19, 20.... they end up being flings that jus pop in and out. By the same token though my good friend Lindsay jus turned 19 and she is dependable. So why would a girl call me, ask me to hang out with her at a later time, and blow me off??? Doesnt make sense to me...