Whatever the cause, this is a warranty item and a very serious safety item. if the air bag goes off while you are driving, that's going to cause a crash and ruin your day. If the air bag doesn't deploy during a crash, that could ruin your life, as you could end up dead.
Get the car back to the dealership and demand they fix the car. If they can't fix it, then in many states you have the Lemon Law to protect you where if after three attempts if the dealership can't fix the problem you brought the car in for, they have to give you a complete refund for the car. Google it.