hi supertony,
yes we have that issue over here in the states too.
the problem is oil in the gas that gets inside the vaporizer and clogs it up. Yes all LPG has a certain amount of oil mixed with it to supply lubrication and odor to the gas.
in our factory training one of the steps in a maintenance operation is to remove the plug out of the bottom of the vaporizer and let the oil drain out of it.
it seems the reason the oil is getting into the tanks is the LP supplier is not draining their own supply tanks or not filtering it like they should so when they fill the small tanks they get a lot more excess oil in them. This is contaminating the forklift system more than usual causing the starting/running issues.
And yes, most of the time they will deny this, once in a blue moon you might find a distributor that admits to it and tries to keep that from happening.
HD5 is the recommended grade of LPG for most machines so that might be something to consider asking the supplier about. (this is usually the customer's responsibility but you should make sure they know it, customers will try to buy the cheapest they can find but i've found quite a bit that the cheap gas is not HD5 even thought the supplier claims it is).