I’m assuming here you’re meaning that the plugin shows up in the Plugin Manager but when you load a vst instrument its not in the dropdown menu? If so, are you using the Default in the plugin collections in plugin manager? This will display plugins in Cubase default manner. If you have made you’re owncollection (like me) where you like to categorise plugins you’re own way, you need to manually add newly installed plugins to that collection. To do so, simply drag and drop the plugin from the left pane into the folder on the right you want (ie. drag Kontakt to a Samplers folder for example).
If you’re talking about stuff not showing up as a Konatkt library within Kontakt, thats a different matter and nothing to do with Cubase.
Cheers,
James