I've had the same problem for some time now, since one of the 2.2 versions I believe. I'm using 2.4.3 now and I'm still noticing the same results.

This is what I've discovered so far:

Firewalls are not responsible for this problem.
Although I'm behind a NAT firewall, I have port forwarding turned on and people can connect. My IP is forced so the network can see it. I use a software firewall too, which is configured to allow connections both ways (from TCP or UDP) for LimeWire. People are connecting successfully, and that's why they're still appearing as valid uploads in LimeWire.

According to the firewall's transmission reports, the very first packet of an upload (8K or 16K) goes out, and then it just freezes there. Push downloads, which also connect in, work fine. This is a transmission protocol problem of some kind.

I'd guess an error was introduced in one of the recent upgrades. Whether it affects all systems or just some, I don't know. What I do know is that my system can't upload anything except at 8K or 16K at a time.