Stop/start can be really finnicky and if all conditions are not met, the vehicle will not shut off. Every car is different, but the most common issue is with your car's battery. If the battery is not charged well enough, or is older and can't meet voltage or load capabilities required by the car, stop/start will not function. If you replace the battery and it still doesn't work, make sure your vehicle is up to operating temperature and before you come to a stop, turn off every accessory. Turn off the heat or A/C completely (turn the blower fan off), heated seat if you have it, radio, wipers if they are on, and any other component you can think of that would consume power. If the car turns off, then you know that you were just using too much power for the car to be able to turn off and still have enough power to fire back up. From this point, turn some things back on and see what the limit is before the car turns itself back on without you releasing the brake pedal. If the car still does not turn off after turning all these features off, then you may have another issue. Also ensure you do not have a check engine light or any other kind of warning light on the dash (ABS, airbag, low oil pressure or other) because this will cause the stop/start to not function
