Dashboards that used to have export buttons in dashboards now have dissapeared.
The exportable settings is probably not enabled.
Go into the configuration for the dashboard widget by going to:
Admin > Dashboards > Find the dashboard > Click Edit at the top
1. Click the edit button (looks like a gear) at the top right
2. You will notice under the widget's title a "Can Export:" section
3. Make sure this box is checked.
4. Save the widget.