the auto switch to petrol is more due to fuel level alerting rather than 'if its not running on LPG' so thats why it won't switch when you have teh LPG issue.

I assume its not running on any cylinders at all from your comment re injectors?

Check the filters aren't blocked ( LPG has whats called 'heavy ends' which collect over time and can block teh filters).

You don't say what make of LPG system it has installed but my first point of call would be an installer who specialises in that make of kit and get them to run a diag on teh LPG ECU. It could be something simple like a fuse on teh LPG system thats failing. IIRC LPG systems are set to fail-safe so any issues means it will shut down rather than run and potentially cause engine damage.

LPG does need a good spark more so than petrol but if its running fine on petrol then that should be enough that it would run somewhat if not optimally.

Water level is only really critical on cold days. teh vapourisors have a warm water feed to assist in teh conversion of teh liqued gas under pressure to gaseous state for the injectors. in reality above 10 C it will happen without any water feed anyway. You could check that gas is being generated by popping off the output hose for a second or two ( Make sure you do this outside with plenty of ventialtion and obvioulsy no SMOKING!!!! . I watched my installer once swap a fuel level sender in a car that teh owner brought in with a full tank despite being told to deliver it empty. basically he popped the opening and teh gas just sat there 'boiling' whilst he swapped it over. IIRC he lost ~ 1/4 of teh tank )