Motor with too little torque

Hi RepRap community,

I recently acquired a Prusa I3 kit and started building it.

Everything went good so far until I got to the motor part. I configured everything and when I first tested, the motors just kept vibrating without doing any effective work. I tried removing the motor from the printer and see if it worked by itself and, to my surprise, it did. I'm guessing that I have some kind of problem with the torque of these motors even though I saw that they are somewhat used without problems by other Prusa users.

I'm attaching two videos that show the problem I'm having:

• Motor not working
• Motor working

For now I'm just testing the X axis to reduce the number of variables.
I'm using a pololu A4988 driver and a 42BYGHW811 motor.
Things I think are relevant:

• I'm using Marlin and I made a configuration change from
  

   #define DEFAULT_AXIS_STEPS_PER_UNIT {80, 80, (200.0*8/0.8)*1, 1178}

  to
  

   #define DEFAULT_AXIS_STEPS_PER_UNIT {53.33, 53.33, (200.0*8/0.8)*1, 1178} 

• Calibrated the voltage coming from the stepper driver to around 0.95V

I got to say that I blindly tested lots of other values with no luck. To be honest I don't fully understand all the electronics behind the motors and that's the main reason I'm stuck and feeling so frustrated.

Any help would be highly appreciated.
Thank you.