We had earlier logic that burndown was updating when User Story status would change to “Accepted”, it was causing confusion for many customers as they would expect to see the progress of burndown during sprint and would like to see the stories progress reflected on chart when it is “Completed”, since “Acceptance” would normally happen at end of sprint.

Thus in our March 2014 Release, we finally made this logic change and now burndown will be updated when user story moves to “Completed” status.