Out of Office Assistant will not work

I have two users who previously have been able to use the Out of office assistant. One is running Windows 98 and the other is running NT. Now it will not work. No changes have been to the Exchange server. One user was upgrade from a laptop running 98 to a desktop unit running NT. The problem still persists. I have had to resort to forwarding their e-mail to a secondary account the is actually sending the reply.