Problem: I have a Player, Percussion 1, with different Instruments. From Glockenspiel to Vibraphone, Triangle, Maracas, Cymbals etc., as seen in the file below.
Flow 1 shows me all of the instruments in gallery view. In Flow 2 I only see the Tam-Tam (with 1 note in it). How can I see all the other empty staves in Flow 2?
Thanks, Jürgen
1.dorico.zip (497 KB)
probably some unfortunate sequence of events…
in Setup click on Flow 2 and deselect Percussion 1 on the left. You will be warned that the “music” in flow 2 will be lost 
Now, with Flow 2 still selected, reassign Percussion 1 on the left and you should see all instruments in Galley view…
I’m not sure what you’ve done, but you’ve managed to remove all of the voices for the instruments other than the tam-tam in your second flow, so there’s nothing that Dorico can show.
I guess this is part of a larger flow, so I would recommend that you make a new flow in your real project, and copy and paste the material from the non-percussion instruments over into the newly-added flow, then finally copy the existing tam-tam material over. Then you can delete the “old” flow 2, and carry on working with the new one.
Hm… Another try. I have Test 1. Same as before, only 1 flow. Percussion Player with many instruments. And one seperate Drum Kit. Then I import „Test 2“, only Tam-Tam, Xylo, Vibes. I klick on „Merge with existing instruments, if possible“ (sorry, I use the German version, I hope you understand what I mean).
/Volumes/Benutzer/jgrimm/Desktop/Test Dorico Forum.zip
Then the result ist Test3… It only merged the Tam-Tam, but Xylo and Vibes are new players. AND: Same as before, I can’t see anything of percussion. Except Tam-Tam…
Ok, Dorico doesn’t recognise that there is already Xylo and Vibes. No problem, I can copy them from the new instruments…. I could, if I would see the old ones. Daniel, as you wrote, I did something to „delete“ the instruments, but I can’t see what… And: If I make a new Flow 3, everything is here again…
Test Dorico Forum.zip (1.34 MB)
It’s been a long Dorico day, but it does indeed seem like you have to create new instruments and copy/paste/delete in cases like this. Otherwise there is no way to activate the multi-player in the new flow… hopefully I’m wrong, but I did relive your experience… 
You didn’t mention that you were importing flows from another project. There are some limitations concerning how players with multiple instruments are handled when importing a flow.