This could be an issue with the free/busy information for each attendee. try using this Outlook 2007 command switch.
Windows XP:
- Click Start.
- Click Run.
- Enter the following in the Run dialog box (with quotations):
"c:\program files\microsoft office\office12\Outlook.exe" /cleanfreebusy
- Click OK.
Clean Free/Busy - Clears and regenerates the free/busy information on the Exchange server. This switch affects Outlook and OWA data, and can only be used when connected to Exchange.
Hope this helps.