So I just tried this out. I believe TRACKING_BRANCH defaults to origin/<currentBranch> instead of origin/master as documented on the rbtools page. That would explain the behavior that we were seeing when we did not specify TRACKING_BRANCH. Pushing to the remote branch on origin caused there to be no diffs when creating the review. Now that I have added TRACKING_BRANCH = "origin/master" into our .reviewboardrc file, now the diffs are created fine.