I've had issues with Transmission. It stopped working and it forced to change torrent client for a while.
I ended figuring out what the issue was. I had chosen a port that Transmission could not bind to. I eventually opened the logs (duh) and was able to correct this but the first thing I did was to look under the port checker in preferences.
The port I was binging to was "green" it was open, but Transmission was unable to bind and it caused everything to stop working, without error.
May I suggest that transmission shows a warning or error under the "Peer Listening Port" check. If its unable to bind a warning listed there would be really handy!
Thanks for the amazing software.
Dr. Sassafras