Why do my coworkers ignore me outside of work?

So over the weekend I saw some of my co workers, I said “Hey what’s up?” to them and they just looked at me and continued talking to each other.
Earlier today after the staff got together and discussed about the next school year, one of coworkers ignored me when I said bye to her. The teacher must of not heard me. I feel like when we’re all together they don’t talk to me or they just avoid me. I know I have autism and an employee but I’m still a person. At least some of them talk to me so I stuck with them.