I don't know, man. Girls are bitches sometimes. Some of them. Its used to be like that for me too when I was working as a security guard in the shopping mall. I greeted info girls and basically everyone but those info girls stopped say Hi to me, still same as you were asking for favors later.
What can I say, it's a cold world out there. obviously, they don't like you for something. External or internal.
Try to not say Hi then. Have to change something in your behavior towards them for things to change. Or bring this up next time they ask for help. Maybe say Hi louder in case they didn't hear you. That also could be the case.
Some people just see you as zero cause you don't tick their boxes in regards to how they evaluate people. Sure it hurts and brings down your self-esteem but as sooner you stop interacting with those people as better for everyone.
As one guy said - you really get to know people(who they really are) once they don't anything from you.
Doubt kills more dreams than failure ever will