help! What does: More sources needed mean?

Sometimes when i try to download some sonhs it says "More sources needed". then i clicked "get sources" and now it says "awaiting sources" but it wont ever download! i have another song that says in the status bar "more sources needed" but I dont know what to do! Someone please help me and tell me what this means and how to fix it!

Thanks!

Clicking on the song and choosing Resume or Find More Sources can help, or if you still have the search tab for that song, command-click on the search tab and choose "repeat search". You can also just try another more specific search.

When a host get busy, goes off-line or other changes, LW can't connect and needs a new list of hosts to try.