Why can't I delete an item in an Outlook 2010 shared calendar when I've been given permission to add, edit or delete?

I've created a calendar in Outlook to be shared with others in my work group (we all use Microsoft Exchange). I've given calendar permissions to certain individuals to be able to add, edit, or delete items. They have been assigned "Editor" status and can add, or edit but not delete. Why can't they delete? How do I fix it?

When I shared other calendars and given individuals editor permissions there have been no issues.