Wow. jb. You've saved me tons of time, that's fantastic. It was as dumb as that. Yes, they were frozen. Again, not being familiar with SONAR, it didn't even occur to me that the clips on frozen tracks would show themselves as referencing mix audio files rather than recording audio files. That's it. Brilliant.
Thank you so, so much. You've all been very helpful, and now I feel like I've wasted all your time a bit, but I'm super glad you found an obvious solution for me. I definitely feel a little dumb on this one.
Best wishes,
Jay