I'm having issues with my beer staying carbonated after I fill a growler. I've force carbonated by a couple different methods -- 30 psi, shaking for 10 minutes, then holding there overnight. Also, hooking it up at 12 psi and leaving it for a week. Same results both times -- it's fine coming off the tap, but fill a growler and when I pour it a couple hours later it's flat. I suspect the beer is just not cold enough when I'm carbonating. (I had to ditch my kegerator when we moved, so it's just carbonating in my basement at 50 degrees.) Any ideas about why it's not carbonating sufficiently?