Do the steppers have encoders on them? If not, the machine can't do closed loop control. It can keep track of where the rotor should be based on counting the steps of coils it has energized, but it has no way of directly keeping track of the rotor. A large enough torque resisting the rotor's rotation can cause the rotor to skip steps, but this doesn't influence the energizing of coils in the stator so the machine has no idea that anything has gone wrong.
An encoder lets you directly measure what's going on with the rotor (and therefore the shaft of the motor). You basically keep track of where you should be (based on counting the steps delivered to the stator coils) then use the encoder to validate. e.g. 'I've sent 12 steps so I should be 48 degrees from where I started. My encoder is reading 26 degrees, so I must have skipped steps somewhere along the way.'