Hi,
From the description I assume this is the PSI 4.3L LPG engine. It sounds like you have checked the basics. You haven't mentioned cleaning the Regulator or replacing the filter in the lock off valve which might be starving the engine.
Combilift have a dedicated Technical Team in the US. If you still need support, you can complete a quick technical support form at the bottom of the main combilift.com web site or direct link https://warranty.combilift.net/technical-support-form/. The form only asks for the minimal info so the team can quickly help you. There is no login required and no charge for this support.

The Combilift US Team will be back to you quickly. Hope you get sorted.