Drive Train Binding
Remove the pinion gear from the motor (not necessary with nitro vehicles) and roll the vehicle on a flat surface to make sure the drive train is free and not binding. For boats, turn the prop(s) by hand. The prop shaft should turn freely with no binding.
If you find that the drivetrain is binding, inspect the driveshaft(s), as well as the center driveshaft(s) on 4-wheel drive models (inspect the belts and pulleys on Nitro 4-Tec 3.3), (or prop shafts for boats) for small objects or debris. In some cases weeds, thin roots, or vines can get stuck between the axle drive pins and molded hex wheel adapters. It is also possible for loose debris to wrap around the driveshaft(s). Floating objects such as plastic wrappers or fishing line can get caught up in the prop shafts on boats. Remove the wheels (or props) and check for debris that may prevent the wheel (or prop) from rotating. On 4-wheel drive models with an aluminum center driveshaft, be sure that the driveshaft is straight and not bent. A bent driveshaft can contact the chassis and bind the drivetrain.
It's also very possible that the motor is binding - not the drive train. Put the pinion gear back on the motor and try to spin the pinion gear. If the motor is binding, give it a thorough cleaning with motor spray and then re-lube the bushings with light machine oil and try again. If a thorough cleaning doesn't fix the binding problem, replace the motor.
If the motor and driveshaft(s) are free and binding is still occurring, inspect the transmission for broken pins or shafts, or stripped gears.