Found a lead: games built with newer OS versions may just not appear at all on devices still running older versions of the OS. You can check to see if this is the case by going to your Settings on the Playdate, then System -> System Update. If your device is behind, that should start the update!
Also note: some of the updates are applied in sequence, i.e. it needs to download and apply them one by one. Even if you do get an update, once it restarts go and check again, it might need another turn!