A couple things you can look at are the vacuum/electric solenoid that is supposed to close off vacuum to the propane lock when the unit is switched to GAS. No vacuum to lock = no propane flow to motor - is the way it's supposed to work when it is switched to gas. To check it, switch to GAS and pull the vacuum line off of the propane lock then try to start it. If it doesn't run there is a fuel delivery problem and I would suspect the wiring at the propane/GAS switch. Check for loose connectors at the switch.

Note, MOST propane locks are vacuum operated but I am aware of some electric ones, not sure how those work but I suspect pulling the hot wire off will lock out propane.