I'm using the latest version of Ubuntu and Transmission.
According to Transmission, the file I am downloading has 0 peers and is idle, even though there are a few peers seeding it.
Here's a batch of error messages from Transmission's log:
> Could not connect to tracker. Tracker gave HTTP response code 503, 504
This is the firewall's status:
> To Action From
-- ------ ----
51413/tcp ALLOW Anywhere
51413/udp ALLOW Anywhere
51413/tcp ALLOW Anywhere (v6)
51413/udp ALLOW Anywhere (v6)
51413/tcp ALLOW OUT Anywhere
51413/udp ALLOW OUT Anywhere
25,53,80,110,443/tcp ALLOW OUT Anywhere
53/udp ALLOW OUT Anywhere
67,68/udp ALLOW OUT Anywhere
5050/tcp ALLOW OUT Anywhere
51413/tcp ALLOW OUT Anywhere (v6)
51413/udp ALLOW OUT Anywhere (v6)
25,53,80,110,443/tcp ALLOW OUT Anywhere (v6)
53/udp ALLOW OUT Anywhere (v6)
67,68/udp ALLOW OUT Anywhere (v6)
5050/tcp ALLOW OUT Anywhere (v6)
Also, Transmission says that port 51413 is open.