Vauxhall zafira 1.8 club engine sometimes dosnt start?

My vauxhall zafira 1.8 club sometimes dosnt start you turn the key the lights and everything go on but the engine dosnt turn over or anything but then other times it starts no problem had it at the garage cause when it first happened a thought it was the starting motor but they said it was fine and they couldn't find any problem with it can you help me