The phone has the YouTube application already installed in the Applications folder, but when I try to watch videos, a black screen appears. I connect to a WireLess Internet Network. The program works fine, I can search and browse videos, but I can't watch them. When I open a video, the time is showing off and it goes by, but there is a black screen and I can't see anything. Also there is no sound. What should I do to make them work?