Zoho & Outlook Integration

Hi. Can anyone help me sort out why only some of my emails sync with Zoho from Outlook but not all of them. I have the contacts correctly set up but if I add received emails only some get added (and are labelled sent instead of received) whereas if I add my sent emails (even to the same contacts) they ALL get added to Zoho. I can't understand why or seem to fix it.

Any help would be greatly appreciated.