It is because they do not fit on the chart width. Try a
greater chartwidth. Alternatively you may want to change the
orientation of the labels in the style of the chart you are using.
Check for this on your style file and change orientation to
vertical.
<xAxis>
<labelStyle isHideOverlapped="true"
orientation="Horizontal"/>
<titleStyle font="Arial-12-bold" isMultiline="false"/>
</xAxis>