Hmm, wonder if you've got a faulty sensor?
As rt10415 says the manual states:
After the engine switches off automatically, it will not start again automatically if any one of the following conditions are met.
▷ The driver's safety belt is unbuckled and the driver's door is open.
▷ The hood was unlocked.
Some indicator lamps light up for varying lengths of time.
The engine can only be started via the Start/ Stop button.
2012 F30 328i Lux', 8SAT, Melbourne Red