I just got back from the Welding shop and they gave me a beer mix of 70/30 and when I hooked it up to my keg and stout faucet, no head at all under 30 PSI. The beer is carbed appropriately and figured the beer gas would do the rest. Do I need to let it sit under 30 PSI for a while? I just assumed holing it under 30 PSI and pouring would produce the typical Guinness head I was expecting. Any advice?