The 3 weeks at 70 degrees
, that we recommend is the minimum
time it takes for average gravity beers to carbonate and condition. Higher grav beers take longer.
Temp and gravity are the two most important factors as to how long it will take.
Stouts and porters have taken me between 6 and 8 weeks to carb up..I have a 1.090 Belgian strong that took three months to carb up.
And just because a beer is carbed doesn't mean it still doesn't taste like a$$ and need more time for the off flavors to condition out.
Everything you need to know about carbing and conditioning, can be found here Of Patience and Bottle Conditioning.
With emphasis on the word, "patience."
Carbonation is actually foolproof, you add sugar, the yeast eats it and farts co2 which carbs the beer. It's not a complex system, and there's very little that can go wrong...It just takes time.....
There really is no other answer than patience, because there really isn't a problem. It really is a simply and fool proof process. The problem arises that we try to govern the behavior based on our timeframe, and not the yeast's. They don't read calendars or instruction sheets, they just do their own thing in however long it takes them.
I've been doing this for years, and bottled nearly a thousand gallons of beer, and have never had one that didn't carb eventually. And I don't do anything special to them at bottling day, that isn't explained in my bottling sticky. You just gotta wait.