Had a problem with outlook 2010 where free/busy of other network users are not showing for specific user. You would go to the calendar, schedule new meeting, select attendee and selected attendee’s calendar would show tentative greyline.
Here is what we did to fix this issue. It might or might not work for your situation but at least a solution for us.
1. On outlook 2010, go to File-> Options
2. Select Mail from the options pane
3. Scroll down and click on Empty Auto-Complete List – Note, this will delete all your auto-complete address when you are sending emails to previously contected addresses.
4. Click Yes to confirm the deletion
5. Restart Outlook