Usually when it happens to a new brewer, it's because they've opened the bottles too soon and they're all not ready yet.
Most of the time when a beer is acting weired, it's just that it's not fully carbed yet. And if you're below 70, or were below 70 for any period of time during the 3weeks, then the beer hasn't fully carbed yet.
Inconsistant carbonation, simply means that they are not ready yet.
If you had opened them a week later, or even two, you never would have noticed. Each one is it's own little microcosm, and although generally the should come up at the same time, it's not an automatic switch, and they all pop on.
But they all will pop on when the time is right.
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.
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."
A tiny difference in temps between bottles in storage can affect the yeasties, speed them up or slow them down. Like if you store them in a closet against a warm wall, the beers closest to the heat source may be a tad warmer than those further way, so thy may carb/condition at slightly different rates. I usually store a batch in 2 seperate locations in my loft 1 case in my bedroom which is a little warmer, and the other in the closet in the lving room, which being in a larger space is a tad cooler, at least according to the thermostat next to that closet. It can be 5-10 degrees warmer in my bedroom. So I usually start with that case at three weeks. Giving the other half a little more time.
Bottom line, it's not that the sugar's not mixed (I find that concept ludicrous if you've boiled your priming sugar and added it to the bottling bucket), it's just that they all haven't come up to full carb yet....Three weeks is not the magic number for finality, it's the minimum
time it takes....