Hi there,
This might occur if you have not properly created your server group.
-On the Manage menu, click Create Server Group.
-In the Server group name text box, type a friendly name for your server group, such as Accounting Servers.
-add servers to the selected list from the server pool, or add other servers to the group by using the active directory, DNS, or import tabs.
-When you have finished adding servers to the group, click OK. The new group is displayed in the Server Manager navigation pane under the All Servers group.
You can try deleting the server group adding it again and see if they are visible.
create and Manage Server Groups
https://learn.microsoft.com/en-us/windows-server/administration/server-manager/create-and-manage-server-groups
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