This sounds like too little current through the motor windings. Too little current could be caused by too low motor drive voltage. Check the motor supply voltage first.
If OK then adjust the current pot meter.
Check the holding torque of the “egg-motor” if this is weak as well the voltage to the board is too low. On my bot I can hardly move the egg-coupler with power on (enable motors from the “manual” tab in the Inkscape egg-bot dialog).
If the “egg-motor” holding torque is OK but not the pen-motor, try swapping the motor wiring so the pen output is driving the egg-motor and the egg output is driving the pen-motor. If the problem moves to “the other motor” the board output is bad, if the pen-motor is still bad the motor (or wiring) is bad.
Chances are you will find a bad connection (or even incorrect wiring) while performing the above swap.
Please give feedback on the progress.
The color of the pen arm motor wiring is brown-green-yellow-red going from the bottom of the board, (Brown is closest to the servo connectors)