Require WI approval precondition not working as expected

I am having a problem with the require work item approval precondition. It was recently enabled in a scrum process to ensure change sets delivered by Team Members also have an approval from a Scrum Master. Some Team Members are unable to deliver, even with the required approval, and get this error message: "The associated work item (#) does not meet the specified approval requirements". If I disable the require work item approval precondition the same Team Member is then able to deliver. I have double and triple checked that the approval meets the requirement (1 Scrum Master approval).

The process configuration is fairly advanced, with several layers of teams and a good amount of customization. I did not create the process so I am not completely familiar with its design, but I believe I have limited the cause of the problem accurately. Has a similar problem been reported? Can I do anything to further debug the situation?

Thanks,
-Dave