Your junk filter has to be trained according to your preferences & this can take a while.
If you're seeing files marked as junk which you think are ok... mark them as not junk. You can also change how strictly your filter works. See the thread below for some info about the filter
Junk Filter

There are heaps of bad files around now, make sure that you look up a video with Bitzi before download. If the file's been rated then you'll get an idea of it's quality & content. Files with heaps of sources but no Bitzi rating are likely to be fakes
How to use Bitzi