It's probably just a glitch. If it continues to happen, try contacting one of those helpful people listed in the About Section. If you really aren't sure who to contact, ask a helpful Mod, and they will point you in the right direction.
This happens, it's just a glitch. Also, I think if you try to leave a comment on some one's profile and it hasn't been 30 seconds since your last comment, it will just show the last comment again, and your comment won't show up. But I'm not sure.