Battery, battery, battery
Folks, this comes down to battery charge. When it's too cold, or too hot, if you've run accessories for a while, if you've boiled off a little bit too much water, the battery won't have "full" voltage, and the system won't let stop/start put you at risk. THE CAR is operating correctly. Check the battery. I'm in the dry hot southwest. Stop/start works flawlessly for me when the battery has been topped off OR when the temperature is in the 40-60F range, otherwise dead as a post. I'm going back to driving through traffic, so I'm personally gonna look at this more carefully, but seriously, check the water level, use a trickle charger.