keep refresing
sometimes it means you've been sitting on that page for too long, someitmes it means that there are too many people watching it at once.
Maybe your Flash Player is up-to-date, but I'd make sure just the same.
Also, if that doesn't resolve the issue, you can use Quicktime to view the videos at the Revver site for Lonelygirl15. Select the video you wish to watch by clicking on it, and you will find a link below the video to watch it in Quicktime.
I've been having to use Quicktime... but I hate it because it's hard to get efficient screen caps. When I get to my computer today, I gotta double check my version of Flash...