It takes time for the video to convert. I changed that upload success message to be one that says please wait, and then set it to refresh the page every few seconds. When the conversion has finished, they will get a success message with a link to their video.

Even with that I still get some people who leave before the video has converted, and when that happens I just get nothing. The video shows up on the recent videos list, but nothing will happen when you play it. Sometimes it can be reconverted in the admin panel, but this doesn't always solve the problem. I check my site several times a day and play every video to make sure they all work. I delete about half a dozen videos a day that members do not finish converting.