I am an all-grain brewer who cannot seem to hit my final gravity on any of my brews. I just finished a stout that should have gotten down to about 1.020 specific gravity even with the roasted grains, but it stopped at 1.037 after two weeks. I am aerating my wort with an air pump and air stone, I am using a yeast starter, adding yeast nutrient, and using a temperature controlled freezer with a conical fermenter. I don’t know what I am doing wrong!
Jason Manzano
via email

This is a difficult question to address because there are some important facts that are missing, such as wort original gravity (OG), grist bill, mashing method, yeast strain and fermentation temperature. But your problem is not uncommon to many new brewers and some general advice may help you with continuing to brew batches of beer that don't seem to finish.
   
I really want to know the wort OG because this has a real influence on the final gravity. A good rule of thumb for estimating expected final gravity is to multiply the number of gravity points in your wort (or °Plato for those like me who use Plato) by 0.2–0.25. For example, if the OG of this batch were 1.080 (80 gravity points) I would expect the batch to finish somewhere between 1.016 and 1.020. If the OG was actually 1.110, for example, the expected final gravity would be 1.022–1.028. If the recipe indicated something different from this range I would question the accuracy of the recipe. In your case, this brew really finished high and there is a problem beyond the obvious.
   
Culprit number one on my list is the grist bill, in particular the special malts that you used for your stout. I am assuming now that you either are an all-grain brewer or you used extract with some special malts added. High percentages of caramel/crystal malts in the grist will certainly increase the finish gravity because these malts contain a much higher percentage of non-fermentable carbohydrates than do paler malts that have been mashed. This is due to the formation of Maillard reaction products during the production of these special malts. If you used large additions of special malts, that could explain part of the problem.
   
Assuming that you brewed your stout using all-grains and no extracts, the high final gravity could be the result of an excessively high infusion mash temperature, too short of a mash at the proper temperature (148–158 °F; 65–70 °C), excessively thick mash or mash pH out of the range of 5.2–5.5. The goal of mashing is the conversion of starch to fermentables and it is very important
to accomplish the goal through careful control of all variables that influence conversion.
   
Then there is the yeast strain and fermentation temperature. It is good that you aerated your wort using a rig that should do a good job, you used yeast nutrients and used a starter for the yeast. These are all things that should help ensure a healthy fermentation. But even when you perform all of these procedures properly, it is still possible to have problems if you selected a finicky strain that flocculates early, has a tendency of hanging up, like some saison strains, or has a particular temperature affinity. I honestly doubt that you have a yeast problem due to the care it sounds like you seem to have taken, but it is certainly a very common cause of fermentation issues.