Fiat 500 Not Tripping Light

I ran into an interesting situation tonight.  I was sitting behind a Fiat 500 at a stop light in the left turn lane and it apparently did not trip the sensor, so we all sat and waited another light cycle (I crept up closer behind it hoping my car would trip the sensor).  This is a common issue with motorcycles, but it's the first time I've seen the issue with a car.  Well, the 500 is a small car.