After changing the MDM sync interval for the Windows Phone client, already registered devices do not synchronize using the newly configured interval.
First seen in
Sophos Mobile Control 3.5
The setting 'MDM sync interval' in the 'Windows Phone client' tab at 'Settings | General' only applies to newly registered devices.
What To Do
In order to apply the new sync interval to already registered devices you have to re-register the device.
Uninstall Sophos Mobile Control from your Windows Phone 8 device (see article 119494 for instructions) and register it again (i.e., using the reconfiguration task available in the Self Service Portal of your Sophos Mobile Control server).