My calipers are not working right. Why is the slider clicking?

[Drex]

Ok there's definately something wrong with my fat caliper because half the time I am trying to take measurements it "clicks" before the 2nd of the 2 ends even touches my skin! I have noticed that the sliding bar is not exactly sliding very easily and it kinda sticks and then jumps ahead on the bar as I try and close it, and half the time it clicks before I even finish clamping my skinfold. So I dont think my readings are going to be very accurate like this. Can I add some WD40 so the sliding bar moves along easily? Or is it supposed to have some resistance as it moves in order for it to work right? I think mine has TOO much resistance as it is clicking before I even have it against my skin half the time