It shouldn't be the ARL.
Have you tried connecting up the motor out of the gearbox to see if it spins? If you do this, make sure to hold it down firmly when you pull the trigger! If it doesn't, try the same thing with a different motor; at least that will tell you whether it is the motor or something else.
Just another point: are the bent over ends of the cables where they connect onto the motor intact? Sometimes, they can break where they bend, which can be hard to spot because the heat shrink holds them in shape.