I've been having issues with my Winchester 96 O/U (plain version of Winchester 101). It failed to fire the lower barrel only, approximately 15% of the time. After 3 trips to an established gunsmith and no cure, I cleaned firing pins, replaced firing pin and return spring, hammer spring - and finally observed that the lower barrel will eject empties without opening the action far enough to #### the lower. Trying to fire the lower barrel gave the same trigger feel and sound as dry firing empty gun.
Try removing the butt stock and firing the gun with snap caps, and opening carefully observing how the parts move.
Bob