How to troubleshoot missing CAM messages

Yesterday on a test photo survey mission, about a third of the hot-shoe CAM messages were missing. All of the TRIG messages (camera shutter commands) were present - about 850 images.

On previous missions of up to 1400 images, none of the CAM messages were missed.

Can someone please offer some suggestions on how to troubleshoot why these CAM messages might have been missed?