I have an IPhone user that cannot get all calendar events to sync. It would appear that calendar items that he has been invited to, show up on his phone.
Calendar events that he has in Outlook and OWA, do not show up on his phone. I can create an event on his IPhone and it syncs with OWA and Outlook. If I remove his Office 365 account from his IPhone, and add it back, the event I just created will not show up on the phone (along with all the others that are missing).