Ever since I upgraded to Snow Leopard a month ago, I've been having Transmission problems. As I had a host of other fixes that needed to be done in my system with the upgrade, I couldn't concentrate on resolving it. I was running the latest version of Transmission, as suggested by this site. I noted the port was often closing on me or not working at all. I randomized it and that wasn't helping. It didn't appear like I was seeding anything. The upload was often at zero k even though I've over a thousand items I'm willing to seed.
This weekend I resolved to get to the bottom of it. I went into my router and made sure it wasn't the problem. I am not using a software firewall at present because all my apps are legally owned although I have used Little Snitch in the past. I called my ISP. No, they weren't interfering with my (nefarious) online doings as they view P2P activities. Last night, I queued up 18 torrents and none would download. Port was closed, I relaunched the app. Port was open but no activity in either direction. For the hell of it, I emailed the torrent file to a friend and asked if he could get them. Yes he could. So I knew the problem was on my end.
I studied the manual for Transmission thoroughly. I'm not a computer geek and know little about networking and don't understand what I read in Apple's Console application but I was resolved to get this working. After considering the entire arc of my system upgrade, I did the one thing I didn't think would work. I downgraded Transmission. I went back to a Transmission build safe for Tiger (1.5.4). And EVERYTHING is working once more. Upload/Download all working with no bottleneck. My upload is a steady 50k which is my allotted speed for it and always was active up till my system upgrade and the move to the newer Transmission built for that o/s. I don't know why, I just know it is. and I just want to put this out there for anyone who is pulling their hair out trying to get Transmission to work on the current o/s and failing.