Here's an issue we run into every now and then so I thought it might be worth a mention here.  If you're noticing GP and software distribution delivery failures then this may be your issue:

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Issue: Collecting large amount of device inventory information during an Open Mobile Alliance (OMA) session may cause the System Center Mobile Device Manager server to timeout.

Cause: During the first OMA session on some devices, if the device is instructed by the server to collect a large amount of inventory information it might result in the session taking up to 20 minutes to complete or it may timeout completely depending on the timeout settings on the server. This delay results from the time taken by the device to retrieve and process inventory information that the server requested.

Resolution: To resolve this problem, restore the default inventory collection set using the Restore-MDMInventoryDefaults cmdlet or remove items from the inventory collection using Set-MDMInventoryItem cmdlet.

Additional Information: In this scenario the device user will notice Group Policy and software distribution delivery failure.  The server administrator may also notice two messages in system logs:

The management session with device xxx (device SID is displayed here not device ID) was not completed. The connection may have been lost, or the session timed out due to inactivity.

and

Erroneous OMA DM message received from device xxx  (device SID is displayed here not device ID) during management session.

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J.C. Hornbeck | Manageability Knowledge Engineer