You could have something under the pillion seat jamming the lock? I had this at the dealers once, it turned out to be the owner manual I'd put back under the seat. IIRC the mechanic pressed down on the pillion seat while trying to turn the key, so you could try a 2-man approach so you get maximum pressure on the seat. Can you see anything jamming it through the gap between the rider and pillion seat, assuming you have the pillion seat in the rear position? One of those cheap endoscope cameras you plug into your phone might help if you have no luck.