Periodically and with no reason, server side synchronization will stop syncing in a mailbox. No errors are generated, just nothing tracks and syncs. The mailbox still shows successful. The only way I've found to find this issue is to look at the date of the last activity created by a user in CRM. The Outlook app should provide some indicator of number of emails tracked that have not made it to CRM yet. This would be a helpful indicator that sync/track isn't happening (kind of like the pending send notification in CRM web client).