When you're logged in YouTube will prioritize your own comments in a video's history for you, but when logged out it won't give them priority—likely resulting in them being pushed further down the page. If you just left a comment, try reorganizing the section with most recent comments first to see if that makes yours easier to find. If you can see your comments on a video when logged in, but not on the same video when logged out, it's also possible that the video uploader has chosen to hide your comments.