Unable to Copy Appointments from Exchange

I have a user that is unable to copy appointments from an exchange calendar to his calendar. Funny thing is, he is able to copy from some exhange calendars. I verified that he has full permission on the calendars, but he receives the following error when he attempts to copy: The move, copy, or deletion cannot be completed. The items might have been moved or deleted, or you may not have sufficient permission. If the item was sent as a task request, the sender might not receive updates. Could not open the item. Try again.

Any help would be greatly appreciated. Thanks.