need more sources is a huge problem: too many people find it easier to take, but fewer do the work needed to give back.
The 'one source' is probably overloaded with requests and is refusing any more. Poor fella.
As long as people don't bother to set up their machines to "accept incoming connections" there will be "need more sources" problems. There are efforts to code for firewall-to-firewall transfers, but for now . . . Telestrial's advice to force ip is correct.
Can your setup accept incoming? Less than a third of users can. see the red lines on
http://www.limewire.com/english/content/netsize.shtml
To find out if you can, see Preferences (OSX) or Tools->Options under bug reports. If it says TRUE, congrats--your machine is not part of the problem.