hi mazuro, when you are cranking a powerful car like a Mb you might need to race the engine of the other car while you crank your engine. i tried to jump my MB from a 12V Kia battery belonging to my co worker as i had left my head lights on. he turned on his car and i tried to jump my MB. all the lights and whistles came on. but when i tried to crank the engine it didnt even make a sound. after he raced his engine a good bit i finally got my engine started.
Another thing, your batteries drying out often is a bad sign and you might need to change your batteries. if you should fill them, do so with with distilled water available at most markets.