Sounds like his mechanic prob checked all the simple stuff and I would hope that included the lockoff. Some Hysters use the Aisan regulator that has the electric lockoff built into it. So without a model or serial number we're kinda shooting in the dark. If it has a distributor cap that at least tells me what it probably isn't.

One odd problem I ran into years ago was I had a Clark that was super hard to start. Everything checked good and it was really throwing me for a loop. One of the older techs turned me onto what it was. Turned out the little relief valve in the propane line (the little brass nipple that's by the tank usually by the bulkhead fitting/filter and usually has a small plastic cap on it) was bad. Replaced it and the truck fired right up.

Word of warning to the original poster. DO NOT try to replace lpg parts yourself. Propane is extremely dangerous. Aside from the obvious fire hazard, the propane in the lines is liquid and under pressure. When it hits atmospheric pressure, it will boil instantly which in turn causes a refrigeration effect. This is several tens of degrees below zero. I've seen more than a few people have to go to the hospital with 3rd degree freezer burns while changing tanks or not being careful when working on LPG systems.

That being said, I always recommend to my customers that they wear eye protection and heavy rawhide leather gloves when changing tanks to give you a few extra seconds if something starts leaking.