Did you calibrate your hydrometer? At the correct temperature? Did you measure your beer at the proper hydrometer calibration temp or apply a temperature correction factor?
Using your numbers ProMash gives 1.009 for an OG of 1.052 and a final reading of 6.4 Brix.
My beers ranging from OGs of 1.032 to 1.055 come pretty darn close when I bother to do a hydrometer sample.