Comments on this site are often automatically held for moderation, and appear after they are approved. Unfortunately, in a relatively small number of cases they get caught in the spam-comments filter. For reasons I’ve never been able to figure out, the software seems to take a dislike to certain commenters and throws most or all of their comments into the spam file, even though I never see anything visibly wrong with the content of the comments and even though the software is supposed to assign positive weight to being a previously approved commenter.
When it runs heavy, comment spam can amount to many hundreds of spams a day, which defeats the hope of sifting through them individually in search of legitimate comments, and means I just wind up deleting them in bulk. When the flow is light, as at the moment, I do try to make this effort, and sure enough two legitimate comments just showed up that had been wrongly marked as spam over the past day or two.
If your comment never posts, even though it’s innocuous or constructive, there’s a good chance this is why.