Car window motor control output
A car window motor is surprisingly beefy for its size : that means it takes quite a lot of current. Which is OK because it never runs long enough to overheat.

Measure the DC resistance of the motor. (Several times, turning the shaft, choose the lowest reading). 

Divide 12V by that, to get the motor's stall current, and edit that into the question. I'm guessing something like 6 to 10 Amps...

As your motor is stalled, it needs that much current to move, which your bridge is not supplying.

Replace your H-bridge and power supply with ones that can supply at least that stall current, and try again.