Get an ohmmeter and measure the resistance between the front and rear pickups. It should be close to zero. Likewise, do the same for the wheel sets without traction tires. It’s likely the wheel set to truck resistance, especially if the truck block has a plastic base. The fit between the bronze bearing and truck block can be sloppy causing poor ground pickup. We used some 0.005” brass shim material formed into a “U” shape to cradle the bearing into to truck block to make a tighter fit. Also, if the ground screws have the black oxide coating on them, run a jewelers file under the head to remove the coating or use a nickel plated 3mm screw. Premier 2/3 rail locos don’t have the best ground pickup. It helps if both outside rails are grounded.