Or spending around 200 bucks (300 bucks for battery charger n tools) for an actual starter hobby grade one that will be
1) Faster
2) Durable
3) Reliable
4) Replacement parts if you break
5) Service-able everything and electronics
6) Can last for years and years of fun, while a cheapo Walmart /...
Do the each motors have gears to spin the tires? If so check the gears and the mesh in-between both gears. If its driven by an axle, check the axle. Other than that I can't think of any other reason. For those kind of cars, once its broke, its broke forever, sadly.
How are the wheels hooked on...