2000 Questions for all other cars starting problems

Hi there
i have Holden Commodore Berlina (which is Lumina) and I have a problem with starting system.
i have already changed the spark plugs with all wires, battery, starting relay and made some minor repair to the starter. It starts normal every day regrdless the weather or the time of the day but for some reason I try to start it again it will not. I hear the noise of the starter when i'm trying to cranck it but it will never start. When I wait 15 minutes or more then I come back and try again it works Okay. It happens so many times when I should wait 15 minutes before restating the car and every time it works. I thought of all the items I have changed so far but it looks no luck. I also used the spare key thinking this will help but same happened tonight. It will not start for some reason and I wait 15 minutes it will start.
please help me on this issue since i;m lost and every mechanics tried his best but no luck.
thanks