Hello,
We plan our sprints in a way where we have user stories (top level) and sub-tasks (tasks) low level.
We estimate stories in ideal days, but we don't estimate tasks at all (so every subtask estimate is actually 0).
However we do log time on tasks, since it's logical to report time to the sub task you work on.
But our time spent on subtask does not seem to influence remaining time on user story level. That is why sprint burndown chart does not work 
How can we fix it ?