When you’re using SQL AlwaysOn Availability Groups pay attention when applying the following patches:
- SQL 2012 SP2 CU3
- SQL 2012 SP2 CU4
- SQL 2014 CU5
The Availability Group may stop synchronizing due to blocking between user queries and a system session. The fix is to disable automatic failover if it is activated, restart the SQL Server instance that is acting as Primary of the availability groups and re-enable automatic failover if it was activated.
A fix will be provided in time and is expected early February.
More information can be found here: http://blogs.msdn.com/b/sqlreleaseservices/archive/2015/01/21/alwayson-availability-groups-may-be-reported-as-not-synchronizing-after-you-apply-sql2012-sp2-cu3-or-sql2012-sp2-cu4-or-sql2014-cu5.aspx.