There are a few problems that can cause your transmission to fail.
First of all, please make sure that your software is up to date. If you are having trouble with updating the program, follow this link.
Transmission problems are often related to your browser. Make sure that your pop-up blocker is off when you try to transmit your return. Clearing your cache & cookies, or trying a different browser has helped others.
If none of this helps, you can contact Support to run through additional troubleshooting.