It seems that with my build the motor that drives the back wheels is not working at all. I've tried unplugging and plugging back into the hub but nothing changed. The build is complete but I have this issue, what should I try next?

Does motor spin at all?

Some pictures (using flickr or bricksafe) could also help

Not the motor does not spin at all. 

You could ask lego support for a new motor, or try completely rebuilding the rear.

54 minutes ago, MAMZe said:

Not the motor does not spin at all. 

So when you just plug mitor cable in hub and start it (via smartphone) it doesn't turn?

When you swich engibes, then I suppose front doesn't work.

The best way is to contact LEGO customer support...

When I switch plugs at the hub the back one still doesn't work, so the issue seems to be with the motor itself.

And when you put this particular motor at the ftont it works or not?

One other thing to check would be if you can manually turn that whole rear drivetrain (Spinning the wheels manually, or something with less gear reduction, possibly), just to make sure nothing is mechanically blocked