Dual-fuel Commodore not starting properly

Over the past year my 2010 dual-fuel VE Commodore has been hard to start when cold. The starter motor turns over, but it takes several seconds for the motor to fire. It's fitted with factory dual-fuel and happens with the fuel switch turned to LPG and when the engine is cold. I have twice left it with my dealer for diagnostic testing and they found nothing. Holden has no LPG specialist in my local area, so I took it to an LPG specialist in Bundaberg and he replaced the fuel converter claiming that it was leaking, but that did not solve the issue. I then contacted Holden customer care and was directed to an LPG specialist in Rockhampton who claimed on diagnostic that the fuel converter was once again faulty, but once again it did not resolve the issue. The vehicle was then left with the LPG specialist for a week and he proceeded to replace the spark plugs as he believed that was the issue and once again it didn't fix it. Can you offer any suggestions as to its cause and what I can do about it?