At the completion of the scan, check the result in the top right result window of the Measurement Tab.
A good scan will have the most NPoints possible for that surface. The total possible NPoints would be every pixel of the camera getting a result which is 1024x1024, or 1,048,576. Not every surface is capable of getting full pixels for the image; the rougher the surface, the more dropped pixels. The only good reason for dropping pixels are near vertical surfaces (since this is a reflection based technique, it cannot collect data from a vertical feature).
If there is large areas of missing data, scan again to try and determine where the Measurement Setup Parameters could be improved.