Front Air Bag Sensor Control problems of the 2015 Buick Encore

My front passenger air bag will not turn on with an average size person weighing 135lbs sitting in the front passenger seat. It seems to only work with someone sitting in it at a weight of over 200lbs. I just bought this car and my local GMC Buick dealership tells me there is nothing wrong with the sensor because the mechanic that worked on it sat in it at 220lbs and it turned on for him just fine. Plus they said they are receiving no error codes on it. There is something wrong with this vehicles air bag when an average size person weighing over 130lbs cannot ride in the front seat without airbag protection. There has to be a fix to this as it's extremely dangerous for a passenger under 200lbs to ride in the front passenger seat while the air bag is off! I previously owned a Toyota and it would turn on the passenger air bag even if my 13yr old sat in it at 100lbs.