From what I understand, Transmission bittorrent client uses the miniupnp library to do it's UPnP port forwarding of routers, however on Ubuntu upnpc (a test client of the miniupnp library) doesn't find any IDG routers, while on another Windows machine on the same networks it works correctly (with the windows binary of upnpc). From what testing I've done, it seems that the M-SEARCH SSDP packet is not being sent onto the network, perhaps a firewall or something is stopping it. How do I try and make sure these packets get though and this works?
I just used upnp-router-control to add a port forwarding to my router and it worked like a charm :)
If that doesn't work for you i would suggest you use wireshark to analyze the network traffic and look where it hangs.