There are some conditions that the Play Store will wait for before installing applications even if they are marked as Mandatory. I believe the devices need to be idle, the app being updated can't be in the foreground or in use, and they must be in a charging state. These criteria are the same criteria enforced by the consumer Play Store since at the end of the day it is the same server architecture powering both. This is one of the primary reasons we continue to install APKs directly onto devices via SOTI packages, circumventing Managed Play completely. If you're trying to install LoB applications of your own then I would suggest the direct APK route instead.