You could try changing their service type to something else.
I'm fixing a bunch of users at the moment where there is something funny going on with the service type that is preventing them from being synched with their resilient controller.
For the particular bit of corruption I am fixing, I am deleting their MDUG, which forces their service type to change to Full, then re-creating their MDUG which changes their service type back again, then they start synching between controllers again when I make changes.