1. Check the data source to confirm it is the correct file in the correct format.
2. Confirm that all data columns are properly formatted and labeled.
3. Insert the data into the chart manually and check if it displays the right labels.
4. Ensure the chart type is the correct one.
5. Make sure that the chart has been properly updated after any formatting changes.
6. Check if the PowerPoint theme applied to the Slide Master is impairing the data labels functionality.