Brewed a 13 SRM Berlinerweisse back in September...tried it in November and decided to let it ride for 6 months to see how it might improve. For some reason this beer has been a bitch to properly carbonate. I just noticed my kegerator temp has been drifting up, but the other 3 beers on gas seem to be behaving themselves. Could gassing it up back in November and leaving it somewhat warm for most of the duration be the culpret? It's starting to pour with less foam, but it is sure taking it's time. I've vented it several times and dropped it to serving pressure (and below). It tastes great, but the carbonation is problematic.
I've noticed that when temp comes up in your fridge you'll get more CO2 out of solution. More bubbles and a very heady pour. I've never brewed a sour but I'd think this is true for sours too. I've also noticed that kegs don't want to cooperate for a few pints after you get your temp regulated. Everything just needs to balance out either by burping the keg or by pouring beer. What are your numbers on your balance? line length, ID of the line, temp of the beer (take a measurment next time you pour a pint) and the pressure you have the keg at?
I don't think it has anything to do with the normal factors you mentioned as the other 3 beers on tap are pouring fine.
