When I send a packet back to the client, I receive a "ICMP Port Unreachable" message in WireShark. The client is on a PSP so there is no firewall on the clients side. I do have a firewall on my PC, but turning it off did nothing. The game I'm developing this for has an official server tool that works flawlessly without anything else other than port-forwarding (which I have done)
The server is able to receive packets from the client perfectly.