Newer LimeWires supply the addresses of their ultrapeers in their queryhits, so you can send the push request directly to the ultrapeer to reduce the chances that a push request cannot be transmitted because their is no route to the target host. Traditional pushes are routed the same way the queryhits came. If only one ultrapeer connection along the way failed, the push request might fail.

I thought about this a little more and there are a couple of other cases in which LimeWire goes into awaiting sources mode.

* a FileNotFound response if the uploader rebuilds its library, unshared the file you are looking for. This should happen rarely.
* an uploader sharing popular files or acting as an ultrapeer that did not limit the upstream may be overloaded unable to respond to any further download requests, so the downloads fail
* a host that thinks it has a direct connection to the internet although it is really firewalled may return queryhits to a firewalled servent. Firewalled servents cannot download from each other.