Do you mean that no fixlets are showing up in your “Patches for RHEL 8” site? Click the “Show non-relevant” button at the top of the console, so you can see whether there are a bunch of non-relevant fixlets in the site. So you can tell whether the site has not gathered, or whether clients are not reporting on it.
You said you subscribed the site…did you also subscribe clients to the site? That would be available on the Site’s properties on the ‘Computer Subscriptions’ tab or by checking the ‘Subscribed Computers’ branch in the listview beneath the site on the left pane.