If your motor/drive combination is not sized right for a given inertial load, then any amount of tuning is not going to make a difference.
The Z axis counterbalance should be adjusted in order to provide the same load in either direction.
But even with perfect counterbalance the Z axis inertial load has to be within the capability of the motor.
Are these the original motors, just new drives?
You could also check the current limit that the drives are set to.
Are the drives matched to the motors?