Regardless of whether you primed with krausen/wort, or the more conventional method we use of boiled priming sugar, OR even prime tabs, 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." 
If a beer isn't carbed by "x number of weeks" you just have to give them more time. If you added your sugar, then the beer will carb up eventually, it's really a foolroof process. All beers will carb up eventually. A lot of new brewers think they have to "troubleshoot" a bottling issue, when there really is none, the beer knows how to carb itself. In fact if you run beersmiths carbing calculator, some lower grav beers don't even require additional sugar to reach their minimum level of carbonation. Just time.
The second issue though is what, njbabs
discussed in his post, the gravity of the wort you primed with. We don't know how much sugar was present for the yeast to eat, and consequently the volume of co2 that will be produced because of it.
But the first thing is to give it sufficient time to carb up. If it's been less than three weeks (sometimes up to 6) we don't even know if there yet really is a problems.
A lot of beer are under carbed at 2 weeks.....And carb fine a few weeks later.
