Not sure if I've understood correctly, but it sounds as though the files with no length might be fakes. Have you tried to download these & do they sit 'connecting' for ages before going straight to 'need more sources'? There are fakes around now which are typically new releases/really popluar, heaps of sources (100+ sometimes). If this sounds like the type of file, stay away from the ones with huge numbers of sources... go for a file with 10 sources or less. Also, vary your search terms... look for artist or album rather than song title & try using the All Types search.

Sorry if I've misunderstood your question