This is elementary, but make sure that you have the vacuum signal to the LPG system. Without sensing intake vacuum the LPG is shut off...this as a safety feature so it isn't dispensing a dangerous quantity of LPG into a confined space while the engine is off.

I have also had problems where I thought I seated the quick screw-thread connector on the propane tank. While the threads were properly aligned, I couldn't supply enough torque by hand, I needed a quick boost from a rag and a channellock pliers. A quick test for this is to back off one of the JIC swivel fittings on the thin propane hose going down inside the machine and see if you get a little hiss, noting that LPG is flowing. Best of luck.