Automatically Process Meeting Request Not Working

I am trying to create resources within our e-mail system so that users can
schedule rooms, projectors, etc. via Outlook. I have created user accounts
in AD for each resource, logged on using each resource's ID, and configured
Outlook to automatically respond to meeting requests (Tools > Options >
Calendar Options > Resource Scheduling and checked all 3 boxes). I've also
given our users Author, and then later, Editor permissions in the Calendar
Properties/Permissions tab. I've also deselected the default reminder check
box in Calendar Options and I've checked the Process requests and responses
on arrival check box in E-mail Options > Tracking Options.

As long as I'm logged into a computer as the resource and have Outlook open,
responses are handled correctly. Sometimes it takes only 2 - 3 minutes,
sometimes over a half an hour. However, if I'm not logged in as the resource
with Outlook open, it doesn't work. If I then open Outlook, any meeting
invitations in the inbox are processed.

We are running Exchange 2000 and Outlook 2003. I've found a few other posts
on various bulletin boards about this issue, but no one has posted a response.

Thanks in advance for your help.