Hard warm start

The engine may get flooded. On a warm engine, left off for 20 minutes, remove the induction tubing, open the throttle body shutter and smell for raw gasoline. If heavy odor is detected, fuel pressure regulator is leaking internally or fuel injectors are leaky.

The engine may get starved of fuel. As commented by Paulster2, connect fuel pressure gauge and watch for a drop in pressure over 20 minutes with the engine off. Now, here's a quick test for low fuel pressure. When engine is off and warm, turn ignition key "on" but do not start engine (you should here the fuel pump for 1-2 seconds in a quiet garage). Repeat this key cycle 2-3 times, then start engine. If engine starts immediately, without long cranking, fuel pressure is seeping. Check the following: weak fuel pump, leaking fuel lines, leaking fuel injectors, leaking fuel regulator on the return side (remove vacuum line with engine off and watch for fuel drips). See this video for more details.