You might just be unlucky, the 1 host who has the file you wanted may have gone offline, stopped sharing the file or might not have had any upload slots free. The best thing to do is search again.

If you see a file that's obviously bad, highlight it & click on the rubbish bin symbol to mark it as junk. Or you can right click on the file > mark as > junk.
Here's some info about using the junk filter
Junk Filter

and this one's an epic but has some good advice
http://www.gnutellaforums.com/showth...ht=junk+filter