It appears like there isn't enough space for all the labels to be positioned. Is the IntelligentLabels property set to true or false? In case it is true you could try setting it to false and trying to manually update the labels positioning, here is a sample :
RadChart1.Series.Appearance.LabelAppearance.LabelLocation = Telerik.Charting.Styles.StyleSeriesItemLabel.ItemLabelLocation.Outside;
In case the issue persists, please, open a support ticket and send us a sample test application, which demonstrates this issue, so that we could have a look at it on our side.
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