Just ONE per side? There should be as many clamps as leafs stacked together. I have the 6 leaf +2" under mine and it has three times the clamps yours does. I remember sometimes when I ordered SS springs, the clamps were included but not installed. Also sometimes they were left out altogether, but they need to be there. The rule of thumb was clamp the front segment and unclamp the rear segment. I'd sure add some and see if it makes a difference. Very low cost and the labor is free. What have you got to lose at this point?
