How did Dumbledore defeat Grindelwald when Grindelwald was master of the Elder Wand?

By law of the Elder wand, does the holder not win every duel it faces, and therefore how could Dumbledore have defeated Grindelwald?

Unless, due to the fact Grindelwald stole the wand from Gregorovitch, mean that he was never truly master of the wand?
But in that case, surely Dumbledore, and therefore in turn Draco and Harry were never Masters of the wand?