Make sure you’re submitting the comment by clicking the "Comment" button once you've finished typing your comment. If you don't click the "Comment" button your comment won't be saved to the forums.
Have you included anything in the comment? Oftentimes someone accidentally clicks the "Comment" button before writing anything in the box. It's a simple mistake we've all made, and if you're not writing anything, there won't be anything for us to post!
Sometimes, it might take a moment for the comment to appear. Reload the page to see if your comment is now showing up.
If you tried all of those and it still isn't working, then it sounds like it's a bug. Send us an email at [email protected] or submit a help request, and we'll be sure to look into it right away.