Burndown chart for curr iteration blank (prev iteration ok)

The Burndown chart for our current iteration is blank (not rendering). The previous iteration (recently completed) was fine. There are 284 work items assigned to the current iteration and we verified they have estimates specified.

We found the help text shown below and followed the instructions ("Update all snapshot data") but did not help.

Can someone please help us?

Burndown
No work items with estimates specified were found.
Why did this happen?
This report presents historical data from the data warehouse. This data is collected by an automated snapshot task once a day around midnight. It is possible that this chart is empty because the task has not had a chance to run yet.
To run the snapshot task manually, go to the web interface. Ensure your contributor has the "JazzDWAdmins" repository permission, and select the "Administer Data Warehouse" link in the left navigation panel. Select the "Update all snapshot data" button. After the task has completed, try viewing this report again.
It is also possible that the selected parameters did not return any results from the data warehouse.

Thanks,
Tim