No accept/decline tabs in Outlook Tasks

I have 2 users with Office 2002 SP2 that are on Exchange and the Accept/Decline tabs in Tasks in Outlook do not appear from a specific user. They do appear if I send them a message, which I have the same version of Office and I am an Exchange user as well. The person they can not view these tabs with is also an Exchange user. Why are these tabs not being displayed on tasks from this specific user?