There are loads of parameters that the car monitors before deciding whether to allow it to operate.
Ambient temperature, engine temperature, battery condition, electrical load (high-drain things like Climate, HRW, etc.) being just a few.
Personally I'd be grateful it isn't working - I had a 123d with stop/start. It needed a new AGM battery in less than 5 years and 55K miles - by the time it was coded it cost over £250 at an Independent! Then before it was 7 years old at less than 77K miles it needed a starter motor that cost over £600 fitted at Sytner (I couldn't wait for an Indy as it was my only car at the time).
After that I just turned it off every time I used it!
Whereas the "normal" battery in my E86 lasted 10 years and over 80K miles - and the replacement only cost £90.
I can't help thinking stop/start is what killed the battery and starter motor - even if it does get you into a slightly lower band for road tax. :roll: