it does sound like a fuel problem but generally if the o-ring gets stuck in the opposite coupler (on the truck side) it doesnt allow gas through period because it cant push the valves to open the connectors and the truck wont even run, (after it runs out of gas in the lines that is).

not ruling that possibility out totally but seems unlikely to me.
checking for fuel delivery to the lockoff is easy enough by cracking a line and turning the truck on or manually pulling the vacum on the filter/lockoff if it has one of those.

if you get lpgas out of the line during that test? then i'd say the problem is somewhere else.

justinm is correct- make sure the female coupler (the spin on coupler attatched to the line to the forklift) is about an 1/8" from the large nut on the propane tank. There are 2 nipples in the tank connection that have to depress one another to let propane flow (a safety feature).

Why was a tune- up done? What symptoms were you having before?

did you just change the tank?
make sure the o-ring from the old tank isnt stuck in the coupling on the hose side

the hose side coupling should have no rubber parts in it

be very careful the LP gas can give you 3rd degree freezer burns
wear heavy leather work gloves and good eye protection when changing tanks or working on the fuel system