Final gravity of around 1020 for big brews seems very normal. The brew I have bottled last week FG is 1019 where the OG was in the 1080's.
What I can think of, (and I am sure I am stating the obvious):
1. Yeast, does your yeast have the tolerance for the alcohol? Even Safbrew yeast has max alcohol tolerance. 2-3 11g packets should have been pitched without a starter into an oxygenated wort that is wormer then room temp with yeast neutralizer (that for OG around 1090).
2. Not all gravity is fermentable, if you did not use extract, then how did your mashing process go? Did you hit and keep the correct temperatures?
ehhh... give us more information... :+)
I have no idea if the following question is going to help you or make me look like one of those annoying know it alls
This question has 2 parts:
Part one: What is yeast Attenuation?
Part Two is: What is the Attenuation of your yeast?