When stop-start is working you will notice a letter 'A' inside a circle come on your dashboard. When there is something preventing the engine from automatically stopping the letter A will have a line through it. Examples of things that may prevent it from stopping are cold engine, low battery, high load on electrics, e.g. heated seats, climate control, heated screen on full blast etc, or in the case of diesel engines DPF regens in mid-cycle.
If your engine has auto stopped it will kick in again if the car in front moves away. This is part of the front assist system. I find it quite annoying that it kicks in even if the car in front is only creeping forward rather than moving off. Haven't found a way to override this yet without either fully switching off front assist or stop-start completely. Not sure if there is a way
