All of the wheels are supposed to turn. The single wheel and the front wheel are driven directly by a gear at the end of the drive shaft. If one of them is not turning, then you need to adjust the wheel alignment so the gear shaft meshes with the teeth on the inside rim of each wheel. If there are teeth missing from a wheel, then it needs to be replaced. The rear wheel is driven by the small idler wheel that makes contact with the front wheel. A spring ensures that the idler wheel makes contact with both the front and rear wheels. If any of the wheels (including the idler wheel) have difficulty turning or appear wobbly, then you should replace the ball bearings (2 for each wheel).