Outlook 2003/Exchnge’03 Calendar sharing

I have Outlook 2003 clients accessing Exchange 2003 on Small Business Server 2003. My problem is that when the admin assistants try to add a new calendar item to another user’s shared calendar they get a message that says ‘Unable to save, file has been moved or you do not have permission…’. However, they are able to modify and delete existing calendar items. The admin assts have been given ownership permissions on the shared calendars. I have even tried giving the admin assts full mailbox permissions to no avail.

Thanks in advance for any help.