In the past 2 months, both the App Store and Trakt have experienced outages.
For some, this has required signing out and back in to Trakt.
For the App Store, you may need to check and ensure the Pro license is currently active on the purchaser’s device - and then ensure Infuse has completed a full sync (to ensure it is up to date with Trakt).
For example, if you happened to get invalid data from the App Store during the outage and that was synced to Trakt, it may affect sharing. Or, if one of the devices appears to be logged into Trakt, but actually isn’t (due to the outage) then that could also affect sharing.
I would probably start by logging out of Trakt on all devices, and then logging back in.