Try completely deleting Transmission, including the "home/library/application support/transmission" folder and "home/library/preferences/org.m0k.transmission.plist" file.
Then reinstall transmission. You will have to reconfigure all of your preferences but if you are having problems this may be your only solution. I would also change the port and port forward the new port just to eliminate any possible sources of your issue.
Also, when you port forward a port, it works much better when you make seperate port forwardings for tcp and udp.