Hi Kev Gallagher,
1. What is the version of the WSUS server , is it on Server 2008R2?
2 Are the security updates synced and approved on the WSUS server for the clients?
3. On the clients, check updates online to test if they can detect updates from Microsoft Update.
4. On the WSUS server, please run Server Cleanup Wizard, on the clients, reset windows update components, check if it could help:
https://support.microsoft.com/en-us/kb/971058
Best Regards,
Anne
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