Since the kit has done the gravity math for you already, the most likely suspect would be your hydrometer. Have you checked the calibration on it? How long are you steeping your specialty grains?
I think my hydrometer is fine. I did check it when I first bought it and it was calibrated correctly, but I will check it again just to be safe. I steep my specialty grains between 150 and 160 for 30 minutes. Could this be it? I've heard some people only steep for 20 minutes.
But in response to stewart194: are you sure you are scaling your recipes correctly if you are using five gallon recipes?
No I don't think so. I'm not sure how to scale them I guess. After you mentioned this I found that Beersmith has a "Scale Recipe" button. When you select it, it does let you change your efficiency. The default is 72% so I changed it to 65%...but it didn't seem to do anything to my recipes.
Are you getting all of the extract out of the cans? It sticks to the sides pretty well if you don't wash ot out with hot water or wort. Also, along the same lines, the syrup can pour all the way through the water in the boil kettle and burn to the bottom if you don't stir while adding it. Also for late syrup additions make sure you stir well, and then stir some more to make sure the syrup has dissolved before taking a gravity reading.
I'm pretty sure I'm doing this correctly. NB kits come in half gallon plastic milk jugs, so I submerge them in hot water in my sink to make it pour more quickly and to get all of it out of there. I always kill the heat and stir rapidly while doing this. Actually my brother brews with me, so one of us is pouring and the other is stirring, but we stir as quickly as we can so we don't scorch the extract on the bottom. I also use a 3 foot stainless steel paddle, and the paddle part is pretty wide. Definitely bigger than the steel spoon I used to use.
That is the funny part, when doing these same kits in the beginning doing partial boils, I hit the correct numbers every time. The only thing that has changed is doing full boils, and starting with 6 1/4 gallons so that I end up at 5 gallons. And even doing full boils now, my final gravity ends up being spot on. Or at least the IPA was...the Irish Red is still fermenting.
Again, if I have to add 2 pounds of DME to every recipe to achieve the correct OG, I'm willing to do that....I just don't think that is probably the correct way to go about it. I don't understand efficiency, but it appears that my system or something I'm doing is not that efficient! I've read that adding Light DME doesn't hurt it, but I can't help but think that doing this is changing the flavor of the beer somehow...maybe watering it down a little and making the flavors of the other ingredients not as pronounced as they should be? I don't know.
And the reason I brought up Dark DME for the Oatmeal Stout is just so that the color stays nice and dark. But maybe Dark DME would affect the flavor unlike Light DME?
One thing I thought of last night, is that during the first 3 partial boils I would just poor all of the wort into the carboy including all of the hop and grain trub at bottom. Now I'm a little more careful and try to leave most of it behind. I've read on here that some people add all of it and others don't, but that it doesn't seem to affect the beer either way. But could this be a factor in my OG readings? Either way, maybe I will make sure that I swirl everything into suspension that does end up in the carboy before taking a gravity sample.