Won't start all the time...

When problem is ongoing if the engine cranks ok, then the starter should be good.How long since a tune-up? Any applicable trouble codes? Do some testing when problem is ongoing. If it cranks good but won't start, have a helper crank it while you visually check for spark at the plugs. If spark everywhere, use a gage and check proper fuel pressure and fuel injector pulse. If the engine is hot wear gloves. In my opinion, those are the basic checks.