Have you checked the propane coupler at the propane tank? The female brass coupler should be about 1/4 inch from large nut on the male half of the coupler. Make sure that there are no gaskets or O-rings inside the female brass coupler. The 2 nipples inside the coupler have to be fully depressed to allow full fuel flow.

Have you checked the fuel lock off? Sometimes (albiet rarely) the solnoid initially opens the lock off, only to have the solnoid fail & the lock off closes. Try by-passing the lock off to see if the symptoms change. Note- even with the fuel line connected directly to the vaporizer & the fuel tank open, there should be no fuel flow out of the vaporizer until the vaporizer senses a draw from the mixer.