You can wait another week or more for all the bottles to come up to speed. 2.5 weeks is NOT three weeks, and three weeks when at 70 degrees is the minimum number of weeks it usually takes for average gravity beers to carb and condition.
It's really just because they're just on the cusp of being ready.
Inconsistant carbonation, usually 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.
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.
The three weeks we talk about is usually just the minimum. Many beers take longer. If you had given them another week, more than likely you never would have noticed.
Just give them more time. They're not carbed yet, because they're not ready yet.
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.