Airlock activity is no indicator of yeast activity. The yeast can still be active.
My big beers (I average 1.075-1.080) usually take 2-3 weeks to get down to final gravity. I get a flurry of activity for the first week, then it slows down, and takes a little while to finish. I wouldn't worry too much, but keep taking gravity readings. I currently have a golden strong ale that started at 1.075, and after 17 days, is at 1.018, and still falling, albeit slowly. Did you use a yeast nutrient?