Hi Clarence Klopfstein,
The RadSlider checks whether there is enough space for the ticks to show and in case there is not, the control does not create the HTML for them at all. For example, in case the width of a tickmark is 1px and the slider will create 100 tickmarks, it will only create them in case its track element is at least 100px wide.
In your case, the track element is exactly 85px wide and the slider needs to display 99 small tickmarks and 2 large tickmarks. As there is not enough space for the small tickmarks, the slider only creates the large tickmarks.
the Telerik team