Currently for private filters admins cannot View or Edit filters thus cannot see the boards associated
Requirement is to provide a functionality so that admins or admin groups can be added to the private filters by default.

Workaround

A site admin can go to the Manage filters system page and take ownership of any filter:
<your site name>.atlassian.net/secure/admin/filters/ViewSharedFilters.jspa

Once that's done they can manage the filter and its subscriptions themselves.