Causes:
1) You are missing dependencies that are not on the DLS or built-in for TS12. Either the session cannot be played, or it requires an expansion set.
2) Some of the dependencies are faulty. Check in Content Manager and try to redownload any faulty dependency.
3) All of the dependencies are okay except they have locally-modified flags. Try to revert these assets, then redownload any asset that did not revert.