Connection to DC motor on Adafruit Motor Shield V2 for Arduino

I'm continuing here on another question that went unanswered, so I'll try to break down the problem into smaller chunks.

I'm following the instructions on starting/stopping a DC motor using Arduino Shield, as shown on this documentation page and this tutorial video. They both effectively explain the same thing and i followed the instructions exactly.

I am unable to start the motor using the dcmotor object. I tried both Arduino Shield channels. At the same time, if I writeDigitalPin(ardObject,'D3',1) motor will start immideately (pin 3 is set to be shield channel A PWM). The same also happens if I connect my power supply to the other shield channel and I use writeDigitalPin on pin 11 (i.e. shield PWM B).

Does anyone have any idea why dcmotor object won't do its job? I'm not getting any error so it's hard to troubleshoot it.

Thank you in advance.