Commodore motor time out

My 2009 VE Commodore factory dual-fuel sedan has recently become intermittently hard to start. The starter motor turns but it takes several seconds for the motor to fire, and sometimes it 'times out' without starting. I then have to turn the start key again. It only happens on LPG. To prove this, my wife drove it exclusively on petrol for seven days and it never missed a beat, but on gas it fails 30 percent of the time. At the dealers suggestion I left it with him for a couple of days and although it did happen to him a couple of times it was too intermittent for him to make any sense of it. Can you offer any suggestions as to its cause and what I can do about it?