I have been kegging my beer for 20 years or so. I currently have a kegerator with 8 cornys, 6 on tap at a time. All at same pressure and same temperature. Two levels. Every now and then I get a beer that just tastes flat. All the other beers pour perfectly and are perfectly carbonated. Right now, two are flat. One has been on gas for 6 weeks, the other 2-3 weeks. I made up a tester with pressure gage connected to a ball lock, measured pressure in kegs. Same (15 PSI) for both good and flat beers. Temp is 45F. One flat beer is on top level, one on bottom. All lines are 3/16, vary in length from 8' to about 12', but no correlation to carbonation. I have four regular Perle faucets, 2 with flow control. One of the flat beers is on flow control, one on regular. FC is set to about full on. This is driving me crazy! Can there be something different about the beer? Or is it perception? Very little bubbles coming up, no head, tastes flat. Others are just what you would expect for a perfect pour.