Why do my comments sometimes seem to disappear or become invisible to me? Am I doing something wrong?
If you use your "back" button to go back to a page, it will be to the version you first opened, without any of your votes, comments, or edits. You need to do a refresh to see the content you added. (Or, navigate back to the page using links rather than your "back" button, but that's not always possible.)
If the comment doesn't appear even after a refresh, it may have been deleted. If a comment is added automatically on your behalf as a result of a close vote, it's subject to automatic deletion if and when the closure actually takes place. This is true even if you edited the automatic comment. And of course moderators can delete comments.