Why have some of our GMBs slowed down in progress and lost rankings?

This could be due to a recent Google update, changes in local search algorithms, or issues with the GMB listings or reviews. Campaigns are rarely "set it and forget it". Often your campaigns will require some adjustments. As a rule of thumb lower ranking GMBs require a lot less traffic than higher ranking GMB's. Start out with less traffic than you think you require and scale up from there.