That's always a pain. My experience in lowering trucks it's always something different. For example spring in the wrong position, ball joint not set properly or worn, alignment problem ( positive or negative chamber) could make it look uneven, worst case a bent control arm, best thing is check the control arm bushings that will definitely drop one side. Make sure you use bright light because it's hard to tell since they are usually full of grease. Look for cracks, missing chunks on the rubber. This considering you have same size and wear on the tires.