This is one of those issues that "WE" have to make sure what is going on ... That person might be online but is not getting notifications because they are away from their tv or they are watching something on netflix or hulu with family ..
Now unless "WE" know for a fact that they are playing online and they are not answering back we need to give them the benefit of the doubt ....
I don't get notifications because I turn them off when I am watching movies which I just remembered about a couple of weeks ago and I was wondering why I was not getting notifications ...
So make sure that they are indeed just ignoring messages and game invites before you give out an infraction or you confront them about it ...