Had an odd problem in a lab environment. The lab was only two Windows 2012R2 core domain controllers, fully patched and up to date and a WSUS server. For some reason SYSVOL was not replicating and I only noticed when I configured a GPO for WSUS and noticed that one of the DCs never registered in the WSUS console. AD replication was fine - a repadmin /replsum did not show any errors.