Complex sentence: In summer of 1926, Ma won second prize by playing a concerto by Nicolò Paganini, but he was dissatisfied with his performance and progress, so he returned to Paris in August of the same year.
Split into simpler sentences: In summer of 1926, Ma won second prize by playing a concerto by Nicolò Paganini, but he was dissatisfied with his performance and progress. So he returned to Paris in August of the same year.