1. Check if the data labels were turned on. Under the Chart Tools section in the ribbon menu, select Layout→Data Labels → Show Data Labels.
2. Ensure that the data labels were added correctly, to the right data. Under the Chart Tools section in the ribbon menu, select Layout→Data Labels → Select Data Label.
3. Check whether the data label type is set correctly under the Layout and Format tabs in the Chart Tools section.
4. If the data label type is set to 'Value', ensure that the correct value field is associated with the data label. Under the Layout tab in the Chart Tools section, select the slanted arrow beside the Data Labels section. Then, select the category value to be displayed in the chart.
5. Use different data label types and formats in charts to emphasize specific values. Under the Layout tab in the Chart Tools section, select the slanted arrow beside the Data Labels section and the Format tab.
6. If the chart is too crowded to display data labels, reduce the density of the data labels by changing font size or employing the Smart Label option. Under the Layout tab in the Chart Tools section, select Label Options and reduce font size or choose a Smart Label option.