Some synchronous replicas are not synchronized
Synchronous Replicas Data Synchronization State
Some synchronous replicas are not synchronized.
This policy rolls up the data synchronization state of all availability replicas and checks for any availability replicas that are not in the expected synchronization state. The policy is in an unhealthy state when any asynchronous replica is not in a SYNCHRONIZING state and any synchronous replica is not in a SYNCHRONIZED state. The policy state is otherwise healthy.
For this release of SQL Server 2016, information about possible causes and solutions is located at Some synchronous replicas are not synchronized on the TechNet Wiki.
In this availability group, at least one synchronous replica is not currently synchronized. The replica synchronization state could be either SYNCHONIZING or NOT SYNCHRONIZING.
Use the availability replica policy state to find the availability replica with the incorrect synchronization state, and then resolve the issue at the availability replica.