Something you can check, remove the screws then install only the 2 front screws and tighten them down. Does the rear portion of the rail sit raised above the Receiver? Next, switch the screws to the rear only & tighten. Does the front of the rail sit raised above the surface, or normal? Do any of the screws seem strange when tightening? Do any feel like they are going in at an angle?

If all else fails, you can use a 2-piece base.