First of all sorry for the late reply!
It sounds to me like that the current received by the steppers is not enough to move them and/or the acceleration of the steppers is too high. Try to give more current to the steppers (if you are using A4988 drivers there is a small potentiometer to tune the output current). If you have tunned the speed/acceleration parameters try to get them back to the default ones.Also check that the 12V power source has at least 5A-7A of current, otherwise the motors may not receive enough current to move the robot.
Hope it helps!Angel LM