FERROLI OPTIMAX HE 31C LPG?

I HAVE INSTALLED A NEW OPTIMAX HE 31C,
THE BOILER WILL NOT FIRE ON START UP, AFTER TALKING TO FERROLI IT WAS SUGGESTED THAT I BLOW DOWN THE SILICONE PIPE TO THE GAS VALVE.
THIS DID FIRE THE BOILER AND CONTINUES TO FIRE EVERYTIME WHEN HOT,BUT AS SOON AS IT COOLS IT WILL NOT RE LIIGHT UNLESS YOU BLOW DOWN THE TUBE, FERROLI SAY ITS THE LPG MIX INCORRECT AND FITTED A BOTTLE OF LPG WHICH IGNITED THE BOILER EVERYTIME, BUT WHEN THE BULK TANK WAS RE CONNECTED IT WOULD NOT FIRE, THE BULK TANK SUPPLIERS HAVE CHECKED THE TANK / LPG / AND SUPPLY PRESSURE BY RUNNING IT ON A LPG BOILER - PERFECT
ANY SUGGESTIONS ????????