Stop/start only works if I stall

would check brake lights come on when press the brake pedal, if no lights then would be something to do with it, as system senses the brake light circuit for reference, same as clutch pedal has same thing,, also to gear linkage is senses by a two way gear shift sensor on front of gearbox on manual trans and common failure, also any sack in gear shift can cause same thing ie dont get enough travel within gearbox selector fork as that sensor is a magnetic type and senses the selector through the gearbox as such,, below link is what it looks like
also the wiring for this can get damaged around the chassis leg harness area, its a ball ache to get at , with bmw own diagnostics tool its possible to live data this sensor and other sensors so see if they work correctly when using start stop, also if a battery is over 3 years old they dont tend to give enough stored power for the system to keep working as they are designed to never allow battery to go flat while sat in start stop as such, its the kind of fault i try and avoid due to time involved and mine field of maybe's,

The start stop has never worked on our Mini Cooper.
Ditto, never worked on our Mercedes B class.

Our garage (agricultural engineers & domestic vehicles) mentioned years ago that "condition have to be right" & that sometimes its a LONG journey before it kicks in, if not disabled, based on either a larger battery needed or multiples (as with the B class) ...suffice to say I think stop start sucks, our only experience of it working was our first ever introduction to the tech in a hire car at a french motorway toll & embarrassingly needing a french guy with pidgin english to help us understand whilst queues backed up around us! ...not great.

So even when we activated it on the Merc it didn't work on 90 mile runs to the airport (for instance) given a clean bill of health by the garage!
Too many variables out there, & if a second hand vehicle might have been deactivated by a previous owner.

Not much help I know, but its obviously a real hit or miss technology at times. (2/2 cars for us) ..incidentally the brand new VW stop start hire car we had with 16km delivery mileage on the clock had to be replaced within days as engine faults cropped up.