Why am I unable to switch to a new, synchronized, mixed audio track in FCPX Multicam

I'm at the final stages of mixing and editing a music video. I added the final audio mix into into a Multicam clip and synchronized it to the monitoring angle and then selected the new mixed track as the monitoring angle; however, when I try and cut to the new track/angle in the angle editor nothing happens. There is no edit mark. It seems like the audio in the angle editor is locked.

This has never happened before. I've always been able to cut between audio tracks.