Topher nailed it. The higher the gravity the longer it takes, especially if you didn't add fresh yeast. I had a 1.090 beer that didn't fully carb and condition for 6 months.
Lazy Llama came up with a handy dandy chart to determine how long something takes in brewing, whether it's fermentation, carbonation, bottle conditioning....
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.