Hi Mal my name is ***** ***** you for your question.
I have a few questions before I can give you a answer hope you don't mind.
You say it's on gas, do you mean petrol or LP fuel?
These are available in LP,Gas, and diesel, I eliminated diesel since you changed plugs.
When it does not start have you checked for spark at plugs?
Have you tried running on ether so we can confirm its a fuel problem?
It sounds like it's either a fuel problem or the hydraulics are not disengaging and its trying to start with them activated, which will make it turn over slow and not start since it has to much of a load on it.
Also make sure fuel shut off has power with ignition on and during cranking,,check voltage to make sure its not dropping out during cranking.
Let me know