This behavior can occur if the scale metrics do not fit the button metrics.
Please make sure to set the proper positions for the minimum and the maximum in the question. To do so, please go to the Detail Settings -> "Slider" tab, click on the input next to "Position minimum" and then click on the slider below to set the position. Repeat this for the "Position maximum".
Should this not solve the issue, please create a pretest URL that leads directly (!) to the page with the slider, and post it as comment.