In the last three year I have brewed several beers, both light and dark, all-grain, and I fermented some of them with the same types of yeast.
I find that for my lighter beers, my calculations are always correct (I use Brewtarget), but that for porters and stouts I always get an attenuation that is about 10% to 15% lower than expected, even though the lesser yield of the dark and roasted malts is taken into account in the recipe.
However, I do not find any equivalent information or questions about this on the web.
Does any one ever encountered the same problem, and if so, do you then always take precautionary measures in the recipe and the brewing?
FWIW, I also brewed a beer that was a mix of a porter and a dubbel, and I did the following:
- Mash low
- Add yeast nutrient
- Try to aerate well
- Use an active starter
And it did work, but for me this is just a single data point.
EDIT: I do not have any problems regarding pH or getting my correct OG.