Most likely the pivot pin was not centered and may have been binding against the handle (hard to see when the scales are installed), or one of the screws on the pivot was tightened more than the other - this can cause the pivot to become seized to one scale. I recommend loosely tightening both sides of the pivot, making sure the pin is in both sides of the handle and centered, and then alternating 1/4 turn on each side at a time to ensure equal tension.