iPhone says "No Content" for movies when choosing "Manually Manage Videos" in iTunes

On my iPhone, I've chosen not to sync videos and music but have chosen to manually manage them. Sometimes my videos just "disappear" from my iPhone (it says "No Content"). When I hook up my phone to iTunes, I DO see the videos on my phone. Can anyone explain to me why iTunes shows videos on my phone but my phone says they DON'T exist?