MP Motor Test only spins two motors (Terminal spins all 4?)

I have a strange problem with my attempts at initial configuration of a new quadcopter, using APM 2.8 flight controller with arducopter firmware version 3.2.1.

In the Motor Test screen of the Mission Planner, Motor A and Motor C spin up fine, but B and D do not. Similar results in the Compass/Motor Calibration screen – raising the throttle fires up motors A and C only.

Here’s the weird part – if I open the Terminal window and click the Connect button then all four motors start spinning wildly as gibberish scrolls across the screen.

I read elsewhere that the Terminal window is not supported by arducopter firmware 3.2 and above. This is fine – Terminal is sending random commands to the flight controller and receiving random data back, I guess I can accept that.

However, the fact that the Terminal window can fool the flight controller into turning on all four motors suggests the controller is well capable of turning on all the motors, so why can’t I turn them all on in the Motor Test window?

Does this suggest any obvious oversights I’ve made?
Tomorrow I will carefully review all the motor-related connections but in the meantime any suggestions or info would be very welcome.