Sync Framework: Why can't I sync all data from tables that were added to the SyncProviders?

Question

Hi guys, I just kind of confused as to why I can't get to sync all expected data in one sync session. When I try to sync to the central database through WCF services, I only get partial sync, then there are still some data left that were not synched. So I still need to resync.

Is there an explanation why I can't get all expected data in just one sync? OR just why do this happen? Why do I still need to resync to get the remaining data that were not synched on the first sync session?

BTW, I also use the Batch feature when synching.

Thanks in advance!