purchased a Bezgar model BZR1 1/10th scale rc truck for my grandson. Truck works fine until it stops when bumping into something Then the throttle wont function unless the wheels are moved by hand. It has a brushed motor. Tried tapping motor to see if maybe the brushes were worn, but nothing. It will only run when rear wheels are manually turned. What could cause this besides the motor. Battery is new and fully charged.