Our vehicle was rear ended and the front passenger seatbelt did not lock or (possibly) locked too late. The front passenger's face hit the dashboard, as well as their knees and feet. The vehicle that hit ours was estimated to be travelling at about 45 mph and we were rolling at around 5 mph in traffic. The failure has not been able to be reproduced by the collision center, the sensor callibration contractor, nor the dealership. No sensors showed any failures and the tension appears to be correct on the seatbelt. We have been unable to determine why the seatbelt failed during this incident and if the vehicle will react appropriately in the event of another collision.