We have a few hundred iOS devices enrolled to our MDM server. They were working perfectly till about October this year. Since October, we've been having random problems with push notifications used to wake up the devices for MDM commands. A few devices simply stop responding to the push notifications and continue to do so till unenrolled and re-enrolled. Push notifications for other applications seem to get through. App Store also seems to work fine. A device restart doesn't seem to help. Push Notification tokens are properly updated in the server too, till the point where it lost contact.
What could be the reason? Any suggestions?
Note: This is not the case where they stop responding after an OS update. That seems to go fine.