Im using a beer gas mixture of 25% co2 and 75% Nitro for my stouts and porters. My first couple beers/kegs were great. First up was a dry stout then a porter. My third and present beer is the same porter recipe. I pre carbonate the kegs with straight CO2 for a week or so then i switch it over to my Nitro tank and serve till its done. The third and present batch started out ok for the first few pints. But now its as if all the CO2 is gone. there is plenty of Nitro to push the bear out but its pretty flat with no head at all. So im wondering if somehow the gas has separated? Or something else? the beer tastes great but the appearance is all wrong.