I just had the same problem with my Cerveza extract reciepie that I bottled. However I know for a fact that my Hydrometer is off by .04 to .05 so I don't sweat it to much. Odd part for that is that my SG came in only .01 from what the kit from AHS said, so it was perplexing to me that it wasn't closer to where the FG was supposed to be, even accounting for the calibration issue i missed FG by about .005. However it was on the cake for 3 weeks and tasted about right for warm uncarbed beer, with pleanty of trub material around the upper area of the bucket.
I decided to bottle anyway. My ferment temps went a little high a week or so in due to a heat wave, so maybe the yeast dropped early. Hopefully I won't get any bottle bombs, but i have never had one yet when bottling a seeming 1.020 beer (granted it is really around 1.016 due to my hydrometer being off). I figure if it is done, it is done, and 3 weeks is not rushing it into a bottle. Also please note I am pitching dry notty without a starter, so maybe that is part of it. Many factors can play into it, could have been the heat increase, or maybe a low cell count, or maybe the wort was not areated enought to get a good ferment, maybe the extract interacted in my partial boil differently than the recipie accounted for, maybe the extract was old or i used purified instead of distilled water, etc.
However I am a firm believer in Revvy's advice on waiting for 3 weeks and bottle, patience seems to be the key. Also calibrate your hydrometer, it may be off like mine, i never expected it to be so off but i have checked it repeatedly and there is no denying it!!