question about 2000 tahoe not starting on the first cranking

[chad64chevy]

my wife has a 2000 tahoe, i have replaced the fuel pump, fuel pressure regulator, crank shaft sensor, throttle position sensor, fuel filter, belt, spark plugs, spark plug wires! anyways sometimes it will start right up and most of the time it wont doesnt matter if its cold or hot outside! i will crank it over for a few seconds then wait a second or two and i can hear the fuel pump humm then it will start and have no problems! what would you guy think it would be?? i thought about changing the fuel pump relay switch to see if that would help at all its one thing i havent done yet!! i know more about older sbc bbc motors then these new electric sensor motors! these newer motors a sensor goes out and about $300 later you find out you still didnt fix it! thats what happened on my silverado i started replacing sensors and the every last one on the list was the problem!! anyways any help would be great, thanks ahead of time!!