Pre made cable or a termination of cable to a connector?

If the termination was made by you, then ensure the shield is NOT touching the dielectric. Also ensure that there is no foil insulation on the dielectric. And the center conductor is not touching the outer jacket.

Otherwise, if the screw on the antenna is not making a change the ground is suspect. The mounting screws could be touching the conductor of the antenna screw to the bracket attached to the vehicle. Make sure the nylon washer is seated properly within the hole of the bracket, and isolated within to top, bottom, and side.