On some occasions it can be that all sources with the file are busy or have turned off their computers. If the number is very high then this should never happen, but if this is a problem with most of the files you try to download then consider upgrading internet connection or version of Limewire. Check to see if you have a firewall as well (see if a brick wall is on the globe symbol at the bottom of the screen). If, as you say, it only happens occasionally then just try another search result.