If your account is hosted on the Exchange server and the Free/Busy information couldn't be retrieved in Outlook 2007/2010/2013. The free/busy information for other users is displayed as slash marks (/). Note: Free/Busy information missing can be easily confused with Outside of working hours. If you see Free/Busy displayed in light-grey blocks, check your working Hours in Outlook.

 There can be several possible causes for that.

  1. Autodiscover record is missing or set up incorrectly.
    You need to make sure that the AutoDiscovery CNAME record has been created for your domain. Outlook 2007/2010/2013 discovers the Availability Service URL using the AutoDiscovery feature in the Exchange environment.

    Read the Knowledge Base article on What Is An Autodiscover Record And Why Do I Need It? for more information.
  2. Default Permissions for Calendar are missing.
    Check the Default Calendar permissions for the mailbox you would like to view Free/Busy information for: right-click on a Calendar > Properties > Permissions. The permissions should be set to Free/Busy time to be displayed.
    default permissions
  3. Calendar is corrupt. Try to start Outlook with cleanfreebusy switch. Start > Run > type outlook.exe /cleanfreebusy