I have an Oktoberfest in my first keg setup. It's been around a week and I can't seem to get the carbonation right. Currently it's at 45 deg at 15 psi and it tastes flat. There is lots of head but it quickly disappears and there are no bubbles in the beer. Its been in there for a week and the first week was at room temp at 10psi. What should I try to correct this?