Calendar not updating on Iphone when items are removed from Outlook calendar.

Recently a couple of my board members purchased IPhones and IPads. And I have set them up the best I can. If they create a calendar entry on the IPhone or IPad it will show up in Outlook right away. However if they or one of their admins log onto their Outlook calendars and remove a schedule meeting the entry will not delete off the IPhone or IPads. I know there is a 15 minute sync on the devices but I have tested them after hours and days and the entries are still on the calendars on the mobile devices. This creates serious issues when people show up to meetings and destinations that have been cancled days in advanced. If anyone knows anything about how to fix this please let me know.