We run ReviewBoard 1.6.3.
When submitting a diff manually (without using the post-review tool), users don't understand they need to submit complete diffs every time. Instead they may wrongly submit several partial diffs as they make changes, meaning reviewers cannot see a complete end-to-end diff, and ReviewBoard does not work as intended.
This causes confusion and affects the reputation of ReviewBoard itself. People will stop using it if they can't get it to work correctly.
There is a stop-gap solution that could make things easier for users immediately: The ReviewBoard documentation must explain how it's necessary to submit a full diff each time and not a partial diff. This explanation could go on the "Creating Review Requests" page: http://www.reviewboard.org/docs/manual/1.7/users/review-requests/creating/
If the correct documentation is in place, local administrators and advocates of ReviewBoard could direct other users to it, ensuring its correct usage.
See also discussion at https://groups.google.com/forum/?fromgroups=#!starred/reviewboard/RosZjgO7FBo