1995 Ford Falcon car start up problems

I have a 95 ford falcon which runs on lpg/petrol but barely ever petrol. As of about october/november last year and through out summer oh hot and muggy days the car refuses to start first time, as in when i turn the key straight away. i can then leave it on the first ignition click so just the radio is playing and then sit there for 1.5-2 minutes and then retry from there and it will turn over and start every time. There is no flat battery sound, its not rolling over, nothing happens on the first time. The starter motor, router, ignition switch have all been changed, battery is charged.... any ideas whats going on????