If you can download other file types & short movies then your LW's fine! The big movies you're trying to download are probably fakes, there are heaps around. Are these movies new releases? They're more likely to be fake that an older & less popular movie.

Stay away from files with huge numbers of sources (up to 100), go for a file with 10 sources or less. Vary your search terms, try the All Types search as well as Video (eg movie title - avi or whatever format you're looking for).
Always look up a movie before download with Bitzi. If the file's been rated then you'll get an idea of content & quality.
How to use Bitzi