• I'd make sure the account under which the cluster service is running on those machines has a non-disabled login in SQL Server. Just being a member of the public server role should be fine.

    I know this was a common problem when setting up AGs (see https://support.microsoft.com/en-us/kb/2847723 for an example of that), and it will also cause failover to time out as you described. I ran into this with a lab machine where the cluster service was running using the Local System account, and the NT AUTHORITY\SYSTEM login in SQL Server had been disabled at some point after setting up the AG.

    I hope this helps!