Use the Reset Sync. Mappings column to reset the synchronization mappings for a particular user. You can reset the mappings for all users from the Gateway tab; see Resetting Synchronization Mappings.
This action resets the stored synchronization information (mapping) for the particular user, which will cause a complete synchronization of all data during the next synchronization. Perform this action if a particular user has problems synchronizing. This action will significantly slow down the next synchronization for that user.