The sensor does more than just notice if someone is in the seat. There are a combination of sensors that can determine if the passenger is sitting in some odd way that could cause the airbag to do more damage than good. If your passengers regularly lean on the door, don't face forward, or sit more on one side of the seat it could be turning off the system. There is also a delay in the system so that the bag is not constantly going between an armed and unarmed state. Take a look at how your passengers are sitting next time, you might be surprised.