For Google Earth maps we can set the label content and visibility for objects such as contours, but we can't control how GE displays those labels. To confirm that the labels are set on the contours, try zooming in to see if they appear. If they do, then unfortunately there is no more that we can do when generating the KML/Z file - we can't force GE to show more labels along the contour, as that is a decision GE makes when displaying them.