Play Mode-Some VST Instruments don't show up

Play Mode- Does anybody knows why some (most of them) of my VST instruments (like Garritan Personal Orchestra 4) don’t show up ? I’m assuming that the reason why UVI / Mach Five 3 don’t show up either is because they both are AU plugins. Am I right?
My Piano Teq from Modart neither shows up. Only Kontakt 5, Vienna Ensemble Pro 6 and curiously Sample Tank.
To solve the absence of GPO4, I was thinking on upgrading it to version 5…maybe it will work with Dorico?

Mike, please see the question “How do I use a different VST instrument or sample library than HALion Sonic SE and HSO?” in the FAQ thread for help.