2000 Mercedes Benz C Class the starter is not turning the engine and then sometimes it does

This problem originally started about a year ago and the local garage thought it was the starter motor but when they successfully jump started it they decided to fit a new battery. For ten months the car has been fine but now the exact same problem has commenced again. I left the battery on charge for 8 hours but the engine didn’t turn for six attempts but then turned after another go. Does anybody know what could be causing this?