Two questions:
Do you have aftermarket bumpers?
How minor? The sensors are up front and need a solid hit on a factory bumper, aftermarket hits would be anyones guess.
Aftermarket bumpers are normally stronger than the cheesy factory bumper, when hit they dont suffer the impact the same and the impact sensor wont go off.
Could be theres nothing wrong @ith your system you just have stronger bumpers than toyota installed.
Airbags aren't just simply set off on a contact switch (like a light switch that's either on or off). They take into account things like speed, deceleration, crash sensors, etc.
So a front end side impact at 50 mph may not set off the drivers airbag.