auto-start-stop button is turned off
accelerator pedal depressed
insufficient brake pedal pressure
reversing then stopping
car is on an incline
a low-voltage battery
insufficient engine operating temperature - a cold engine is inefficient and hence start stop defeats the purpose of warming it up by stopping it
cabin temperature not at user's target temperature - ie. air conditioner or heater not yet adequately reached target temp
an improperly closed hood or door
an unbuckled seatbelt
ABS sensor fault
old battery
dirty battery terminal
faulty battery sensor
water in the fuel tank
clogged diesel particulate filter (for diesel engines)
check engine lights or codes activated
a disconnected engine component
need for a TCM software update