Hemuset

I have a 2013 Honda Civic. Occasionally when you have been driving it and switch off the engine to pop into the shop it won't start. The engine is cranking but its not catching. If you wait 30 minutes after switching it off it will start no trouble. It has been to the Honda dealership to go on the diagnostic machine but they couldn't find any fault. If I have to switch the engine off i worry it won't start and i also can't use the stop/start as a result. Does anyone know what the problem might be?