Display in display group missing schedule

I have a custom 1.8.11 build on windows server 2012 r2 . Clients are running on PC’s , using mainly 1.8.10.
I have a weird issue in that I have had 2 displays not displaying the scheduled content and reverting back to the default layout.
I have setup display groups and then I schedule against the display groups. I have had 2 displays in this same group revert back to the default layout, all other displays (6) that are in the display group are still presenting ok. The problem displays are still checking into the CMS ok and any schedule that I assign to the individual display works fine.
Have removed and added the displays back into the display group, though still get the same result.
Took display out of any other display groups, still had no affect.

Any ideas