At risk of drift, I had a Mazda 3, 2.2litre diesel and stop-start would only function on that (by design) if the engine was hot enough, so on short journeys it would rarely operate. Could be worth perusing the operating manual to see if Mercs are designed similarly.
The Mercedes stop start is some times subject to what gearbox mode you use eg economy, in the applicable modes the stop start will always be on and you have to push a button to deselect it, mine stopped working and it required replacement of the aux battery and the stop start circuit board replaced. Many ECU remaps will deactivate it permanently.
