You could have a problem with-

1) engine ignition timing. If the timing is retarded, engine vacuum will be very low while cranking, not sending enough of a signal up thru the carb to the regulator, therefore not "opening" the regulator.

2)vacuum leak at carb or intake manifold. Again this will lower vacuum to the regulator. Spray starting fluid around carb & intake manifold while cranking to check this (make sure the air filter assy is in place while doing this- don't get either in the air cleaner assy)

3)dirty seat in carb where the diaphram seats. If diaphram was replaced but seat wasn't cleaned, the residue from the propane is sticky & will keep the diaphram from opening.

4) type of propane being used. Forklifts require grade HD-5 propane. Sometimes the grade of propane isn't suffencient to fire when engine is cold.

5) spark plug type/gap. I've had trouble with mitsubishi engines when it comes to spark plugs- they don't seem to like Champion or SMH spark plugs- I've found A/C or Autolite plugs work the best.

Hope this helps