If you watch Poindexter's video on time lapsed carbonation, you will see that in many instances, before a beer is carbed it my gush, that's not from infection, or mixing of sugars, but because the co2 hasn't evened out- it hasn't been pulled fully into the beer. Think of it as there's a lot of co2 being generated and most of it is in the headspace, not in the beer, so there's still "over pressure" in the bottle, so it gushes when it is opened.
But when the beer is truly carbed it all evens out, across the bottles.
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.
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. You have green beer.
Temp and gravity are the two factors that contribute to the time it takes to carb beer. But if a beer's not ready yet, or seems low carbed, and you added the right amount of sugar to it, then it's not stalled, it's just not time yet.
Everything you need to know about carbing and conditioning, can be found here Of Patience and Bottle Conditioning.
With emphasis on the word, "patience."
Additionally once the three weeks or so has passed, chiling them down for a few days (not just a few hours or over night as most new brewers want to do
) will help the carbonation settle.