also check the LP filter.
and make sure the hose fitting to the tank is all the way home.
we have seen several tank fittings not letting the valve open on the end of the hose..
common issues on LP fork trucks.

have you tried to manual purge the demand regulator?
there is a button on the unit for prime.
you should hear a hisss when the button is depressed.

from there check the spark and spark time with a time light.
and check the compression.
if it is over 10 years old
check the time belt.
if it is cracked, replace it asap.
it might be broken.

these engines are basic.
fuel, air, spark

I read your problem are you sure the lock is feeding fuel to emco converter could be stuck internally. on primary side of converter you will see a plug approx 3 /8 this is were you check the primary pressure you could open and if you don't hear gas lock is not opening if you have a gauge you could see if you pressure