Me too. I get frustrated by forums where old threads are routinely closed, or where participants are rebuked for bumping an old thread.
Or forums where you post something, arguing why open discussion of a certain topic should be allowed, and your comment is rendered selectively invisible a few minutes after you make it.
I often feel guilty for ignoring other people's comments or questions, and frustrated when other people seem to be ignoring me. If I can't indicate to someone exactly why I'm not answering, or can't receive such an indication myself, I can at least help my future selves and others obtain a better probability distribution over such reasons. To that end, I'm listing all of the reasons I can think of for someone to not respond to a comment/question, to save the effort of regenerating these hypotheses from scratch each time and prevent the possibility of failing to consider the actual reason. Note that these are not meant to be mutually exclusive.
If I missed any reasons (that happen often enough to be worth including in this list), please give them in the comments. See also this related comment.