There’s a comprehensive list of inhibiting factors in the handbook. Not heard of (or experienced) “after reversing” and I’m not going to try driving without the seatbelt!
Diagnosing it isn’t helped by the incorrect indications on the display, for example:
1. When ventilation is directed to the screen, you get the A/C symbol as the reason for it not working. In fact stop-start works perfectly well with the A/C if the vent is directed elsewhere (that's manual A/C, not climate control).
2. In cold weather I get the low battery indication as the reason, which is rubbish because I know the battery and charging systems are in good condition. There is a criterion of “low battery internal temperature” which I think the car interprets as low voltage so that’s what the display shows.
I find it works perfectly well through most of the year, but hardly at all from about now through to March or April. From that I’m guessing that the biggest factor is ambient temperature, even in typically mild UK winter conditions, which can of course affect the car in all kinds of ways.