Ive brewed similar beers with 1056 and had more attenuation. I am surprised it stopped at 1.012, my experience with that stuff would lead me to expect more like 1.010. On the other hand, you were using extract, and i always found extract to be on the less fermentable side of the spectrum.
Fermenting: 20 gallons of IIPA
Keg/bottle: Tearjerker IPA, Experimental pale ale, crappy tasting ESB, wonderful Belgian pale