Sometimes people do it without thinking, and it’s nothing personal, they just get really involved in their own plans of the moment and forget everything and everybody else, especially if you’re not with them very often. You feel like you have to jump up and down, waving your arms and shouting to remind them you’re still there.
They might have lots of changes going on in their lives, too, like going to college, or finding jobs. I remember something similar happening at one time. Two friends started spending all their time with their boyfriends, another one was busy studying to go to University, and another kept arranging to meet up, then not turning up. I found out later she had depression and felt too bad to go out. They weren’t deliberately ignoring me or choosing one person over another, they just had other things that took over their lives. Some snap out of it, others will be replaced by new friends!