Propane question:LPG pressure drops down after first stage regulator?

A question for "Propane Guy". I have a 5 ton steam boiler using LPG. when we increase the demand say from 30% to full capacity the pressure after the first stage regulators (3*fiorentin hp100) drops down from 500mbar till it reaches 0. we are using an lpg vaporizer rated 160gal/hr. What do you think the problem is? the regulators are undersized (they are well calibrated)? or the vaporizer is undersized? if burner capacity 3450kw