They may be getting marked as junk. Try going to Tools>Options>Filters>Junk, and put a tick by one of the "Display Junk" options, then click apply.
Also make sure you have "All" selected in the columns to the left of your search results.
Also see:
Fake files showing up in search results How to find music (also tips for video)
Try the suggestions in this thread for blocking bad ip's:
Problems with the search