Pitch yeast a second time

No need to pitch more yeast. 1.020 @ just over 9% ABV is just fine for a beer like this. No reason to mess with it.

Which yeast strain did you use, btw?

 

On what do you base your expectation of 1.012?

 

As others have said, leave it be. You’re not always going to hit FG on the nose, close enough. Actually, for an imperial stout you are going to want the extra body from a higher FG. How long has it been fermenting?

 

One of a few things. The yeast might want a refresher because it is a larger beer. i.e. shake them up, or rouse them and feed them something they can chew on. Or. Ignore it. I give my yeast a shaking on big beers because they hit a spot where they aren't too exicted about crossing a gravity threshold. The simple chains are long gone. Now it's a boozy space, and they have big math to comprehend. Regardless. My guess is, they will probably figure out a way to hit your target fg, or come a few points North of it soon enough if you give them time to.

 

FG estimates are just a SWAG, even from a computer program.

 

Beersmith is quite popular, but its attenuation calculations don't consider the fermentability factor that's dependent on the mix of grains. Stout worts tend to not be very fermentable, because of all the roasted and crystal malts. I haven't seen your grain bill, but it's more than likely that the recipe has no chance of getting down to 1.012.

Some are SWAG-ier than others!