There are a couple of areas of adjustment with Shark S1 Sliders that can affect your shots.
1) Main Belt Tension - If you add the extension rails, or change the belt for any reason, make sure you tighten up that belt. This involves thumb screws and adjusters on the bottom side of the Shark Slider. Refer to the attached instruction - Section 1 - Install Drive Belt:
2) Adjusting the tension of the pulleys (Rail Pulley Tension) - Section 7 of the attached documentation