When you create a meeting in Outlook, you can open the meeting, go to Tracking and see who has responded.
This works fine initially, but if you change the time of the meeting, the tracking of responses no longer works. Even though people have responded to the new time, it just shows 'none' under the Response column when you go to Tracking.
Request responses is checked under Response Options.
Also, in File - Option - Mail- Tracking, the "automatically process meeting requests and responses to meeting requests and polls" is selected, as is "auto update original sent item with receipt information"
This has been reproduced on different computers and happens in OWA also.