Computer Hope

Hello Dear Fellow Thinkers,
I am currently making a movie with Windows Movie Maker. I already know most of the features and applications, but clipping a video clip is wrong. I place the marker pointer on the center of the video clip and cut with the scissors. If I do this, all video clips disappear into the background (almost invisible) and only the black marker will remain. What am I doing wrong?
Grtz
Gilberto