Gs650gjosh

I'm sorry, I know nil about bike mechanics The starter always makes a healthy starting noise but 50% of the time the engine doesn't kick in

Hi everyone, I have just passed my bike test, I've got a gs650g 1981. I love it. The only issue is, its hit and miss with the start up. I've got a brand new battery, starter motor seems fine. What should I else could it be? Cheers