Windows Movie Maker problem

When I attempt to trim the beginning of a clip I often experience this problem:

If I trim too much off in the beginning, the thumbnail of the video in the
timeline or storyboard turns green and when I preview the clip, the video is
gone (only audio left). This happens when I try to trim off more than a few
seconds in the beginning of a clip.

Has anyone else experienced this problem? If so, how can I fix this? There
is often too much at the beginning of a clip that needs to be trimmed to
leave it as is. I've also tried trimming the clip in a separate program, but
then Windows Movie Maker refuses to import it.