Mobility sync issues after moving user to new PO

We have lots of issues whenever we move a user with a mobility account from one PO to another. I haven't been able to pin down where things go wrong. The Mobility Manager interface says the devices are checking in and syncing yet they don't get any new mail. Sometimes we can get mail to sync but no contacts or calendars. Often times we have to resync the account, remove and recreate devices, sometimes it's all of the above. The manager does show the user in the new PO, the logs rarely show any errors.

Where do we begin looking for answers?