Automation pan issue - please help

Hi Guyz

Strange and serious pan issue last night
Using Cubase for live show playback-everything set up correctly and working well EXCEPT…

I have about 20 stereochannels involved in playback…
they all show C (centered) in the mixer window
But three of them play back to the group fully left…

Just touch the centre pan position, and playback is correct.

But after a stop/start, the condition reoccurs…

Any help?