Hugh I believe you're right. I think I'm getting to the root of the problem. The issue appears to be the correct playback device. After checking the mixer, the bass line instrument was being triggered in Sibelius Sounds, not Kontakt. Once I fixed that, it worked fine.
So I went back to the indoor score I'm working on, and while it's better, it's not 100%. When attempting an instrument change from Metal Combo to BD/Tam Tam, the only sounds I get are a generic crash and generic tom sound. I can get the correct sounds if I create an entirely new instrument and manually select the BD/TamTam as the instrument in the mixer panel. Once that's chosen, I can hear the correct sounds being triggered appropriately. And the correct sounds remain that way even after switching the instrument back to (Auto).
So I'm quite certain it's something in how Sibelius is assigning instruments, but I don't know how to fix it and keep it fixed, without manually changing the instrument in the mixer. And that still wouldn't alleviate the instrument change issue.
EDIT - I've got 4 instances of Kontakt and once instance of Sibelius Sounds loaded in the playback config.