Turbo is correct, its probably the spring. Gotta open her up and reposition it. Did you get it from Mato Toys? He will not give you the run around, Murphy provides excellent customer service.

First you have to push a pin out of each sides track to get them off. Then remove 1 screw from the underside of the front hull and two from the rear. Then get a small screwdriver and remove the first two screws on each underside of the sponsons. Slip the screwdriver between the roadwheels. Disconnect the air hoses on the rear deck and gently lift the upper hull off. Disconnect the antenna wire and the connector for the harness. Then you'll remove the 3 screws from the bottom of the turret gear, now the turret will separate from the hull. Then take out all the screws on the bottom of the turret. Be careful with the wiring, it can break easily! Have a soldering iron handy in the even this happens. The spring should be wobbling around in there somewhere. There are tabs to keep it in place both on top of the airsoft unit and the ceiling of the turret.