Some users cannot see room calendar items in Outlook

I have several Room mailboxes in Exchange 2010 for which users cannot see calendar items. Everything works fine for the room delegates, but not for other users. They can see all calendar items in OWA, but none in Outlook 2010 or 2007. I've tried rebuilding the mail profile, including deleting all cached Outlook data, and I've tried both cached and online mode.

Any help or advice is appreciated.