I finally had a chance to look at why the feedback service was working for some people and not working for others.
It turns out that I was pulling the next user to process from the database incorrectly. The way it was working, I'd always be pulling out new users or people who had more recently been updated instead of looking for the oldest person. So, anyone who was a beta-tester or signed up for the feedback service in the beginning has probably not been getting updated regularly (or at all).
This has been fixed. It will probably take a good 36-48 hours to now get through everyone and get back to it's normal 24 hour cycle of processing each user.
Let me know if your feedback service starts working as it should!