Some mobile devices that are known to be missing one or more required apps do not appear in the "non-compliant devices" report.
Before a mobile device can be checked for non-compliance, it must first be targeted in Mobile Management as being a device that requires specific apps to be present on the device. To do this, you first create a library feed to provision one or more required apps. You then target the feed to the specific devices or group of devices that you want to have the required apps.
For more information and instructions to create a feed, see Managing the Mobile Library in the Symantec Mobile Management 7.2 SP2 Implementation Guide
To make an app "required", when you set up the library feed for an app, set Priority to Required. Once this feed is targeted to the devices, the compliance check is enabled and any devices that do not have the app specified by the feed will then appear in the report as being non-compliant.