For any manually installed SCOM agent, you should manually installed UR4 update Patch. Go to Monitoring workspace --> Operations Manager --> Agent Details --> Agents by Version to check whether the agent is successful apply the update Patch.
1) SCOM Agents were installed Manually to 20 Domain Controllers but only 5 of them have SCOM UR4 Update Patch applied.
Any error message displayed when you apply the UR4 Update patch.
Roger