I'm not sure if the engine cranks over and wont start, or if the engine doesn't crank over, but here is a possible answer for both situations.
Cranks over wont start: This is may be caused by the fuel pressure check valve, or an electrical component that is failing when it is hot. I would check the fuel pressure, and then make sure the system is holding pressure when the engine is warmed up, and turned off.
No Crank: This may be due to a bad starter or something in the starter control circuit.
No matter the symptom, have it checked out when the problem is occurring. This is known as a "Hot Start" problem, and as long as you can duplicate the condition, it should be fairly straight forward for an experienced mechanic to diagnose.
Good Luck and let us know what you find out!