Since you've replaced the motors then that's not the problem. Each wheel has a switch behind it. Each switch tells the controller to do something. I'm betting one of those switches is bad. You can depress the pin manually on each of the switches to see if they're working. There's a metal lever that depresses the pin. Possibly one of those is broken off or the switch itself is bad. With some trial and error you'll figure it out. Good luck!
If I can't fix it, it's broken!