If I said hello to someone and they didn't respond what could be some reasons they are ignoring me?

It's possible they are in deep thought and didn't hear you. I have done this myself when out shopping and someone that knows me calls my name. When they catch up with me they laughingly tell me I'm a snob! LOL Give it another shot again and if they ignore you then don't worry about it and don't bother acknowledging them any longer.