I know that this may come as a rather late reply, but there is another factor to consider that most people are completely unaware of…. The ever so dreaded issue of the port forwarding in the NAT not being setup sufficiently.
See... On most internet connections, you will have a single IP address (usually a /32 subnet), which establishes your link to the internet, and then you have your private local /24 subnet (usually 192.168.1.0/24). What happens in your modem is that your computer gets the IP address of (for example) 192.168.1.100, but your internet IP is something like 10.206.173.2. When the request for an internet link is sent to your router (usually built into the modem), your router converts the IP address of the source machine from 192.168.1.100 to 10.206.173.2 before passing it out to the internet. This is most common and is the basis under which most internet connections work.
That being said however, when users attempt to connect to you on 10.206.173.2, unless your modem knows specifically that the port that the user is attempting to connect on (both TCP and UDP) need to be forwarded to 192.168.1.100, your modem/router doesn't know what to do with it and thus drops the connection. When this happens, it makes it extremely difficult to use the more passive approach of your torrent client announcing your presence to the tracker to announce to other peers, then having your computer send the initial signal to the other user's computer in which to use the follow-back path to be able to connect to your computer, despite the router not knowing specifically to forward that port. This can cause some serious communication issues with our tracker, which may result in seemingly being unable to seed at all.
I'm not sure if this particular case applies to you or not, but it's worth a look to see. In general, you can find the port number that your torrent client uses in your torrent client's settings. This number will vary from one computer to the next, so it's important to know which specific port that your torrent client uses to make sure that said port number (both TCP and UDP) are correctly forwarded to your computer. This will allow other torrent users to be able to connect directly to your machine, without running into the issues with having to pass through the NAT to accept the connection. In general, you can find information on how to setup the proper port forwarding on your computer at http://www.portforward.com/ however some of our staff may be able to assist you with getting this looked at if you are not able to do so yourself.