Although this video must be for an earlier version of Dorico, it did help me figure out something that works.
I was assuming (or thought I understood from other videos) that when you assigned an instrument to a player in setup, Dorico would automatically assign some kind of sound to go with it for playback. It appears that’s not the case and that you have to manual assign sounds and channels for each player.
I had a frantic wrestling match with Dorico, but I did finally figure something out to get what I needed.
In the mixer, there is an “Unused” button. If you hit that, do the correct faders show up? Ultimately, re-applying the playback template should cure all.
There are some VST’s that output only to tracks 1 & 2 unless one can configure them to send output to different faders. Dorico may create a fader for each instrument or Player, but that does not mean VST’s will use them automatically.
On occasion, reapplying the Playback Template will reactivate the channel assignments.