Though I don't have an answer (sorry) I wanted to add that I have been able to add multiple TV Shows and Movies just by dragging and dropping. I'm not sure what this could be? Do they all import one-by-one without any problems?
I am not experiencing any problems dragging/dropping movies into iTunes personally. As John posted, I can drag multiple files to iTunes and they all import successfully. For clarification I am using iTunes 9 on OSX 10.6.1 - if you are using iTunes under windows then I do not know what the problem might be... I can only recommend removing the iTunes application and plist files (registry keys?) and try reinstalling it. Please remember to backup your complete iTunes library and xml's just to be safe...