It could be the IP address or the port number. Is that a private IP address or a public one? In doubt, try the standard port number. What I meant with "exact" matches is the filename. It's rather unlikely that a legit reply matches the search term bit-by-bit. Try a random filename instead because might the receiver might consider such replies spam. Maybe LimeWire ignores small files. If possible try to capture a valid reply in the wild and just use that with GUID corrected of course. I could imagine there's something wrong
with your layout at the end of the packet which is ignored by gtk-gnutella but not LimeWire.
Provide a hexdump of packet you send.