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