Go through the following for resolving Outlook sync issues: 

 1: Modify  Profile Settings to Sync Outlook - Your Outlook will be properly synchronised if there are enough items stored in the Server folder to  carry a section together with that of the Offline folder.

2: Re- Synchronize Outlook Folders

Any frequent issues with Outlook can be fixed immediately by re- syncing your Outlook folders. 

3: Empty the "Sync" Issues/ Local Failures" Folder-  If you are unable to synchronise any of the Outlook folders, then attempt to first empty Outlook's Sync Issues\Local Failures folder.