Craftsman Briggs and Stratton platinum series 7.25 hp won't start after running

Hi first time posting on here. I own a Craftsman Briggs and Stratton platinum series 7.25 hp and it just started to do this. Mower starts up first pull when it hasn't been ran for like a hour but after running for a little bit it won't start. But if i grab a different spark plug and put it in there it will start first try. I'm thinking maybe coil? Please help me out I'm stump in this one 😔