It's hard to say how common this is. It shouldn't be common. But I've seen it happen enough times that I would say it isn't uncommon either.
The pattern I've noticed is that those students who are ignored usually are the one's that haven't really been actively close with their advisors. This isn't always the student's fault either. At my school, many have full time jobs and are pursuing their degrees part time. Many have family obligations. Etc. But the fact is that they might be spending less time in the lab, or participating less in the professor's seminars or the prof's work. They might be making slow progress. These students, naturally sort of get pushed to the outside of the "inner circle." When it comes to prioritizing who gets attention, it ends up being those who have a strong relation with the professor who get that attention. Sometimes though, it does seem a little more on purpose. I've seen professors snub students that they don't seem to like - almost as a message to the student to find a new advisor.
Not sure if this is part of the question (between the words) but if you don't want to be ignored by your advisor, there are a few things I think I might say.