The temp is on the higher end of the range given so if anything it should have fermented more aggressively. Also it is a high attenuating yeast so I don't think that would be the cause.
What about calibration of your hydrometer? Do you have some plain (preferably distilled) water stored in the same spot as your beer? If so, take a measure of that to find what your adjustment would be and apply the same to your beer's gravity.