I've been using a new 10 gallon igloo mash tun for my beers this year, 10 in total so far. With belgian pilsner and maris otter, I'm getting great pre-boil efficiencies of around 80-90%. However, in about half of the brews I've used American 2-row, 2-row with 6-row, or 2-row with American Wheat. Two times I performed a step mash that included a protein rest at around 128F, plus the normal Sach rest. sach rest temps have been as low as 148, and as high as 158F. But I always have been getting 60% pre-boil efficiencies. The mash tun is never filled to more than half, and the only beer I did that really filled it was a belgian strong, in which I used belgian pilsner. Assuming pH, water chems, and mash times are all the same more or less. Would stratification of the temps in the layered mash make an issue with American 2-row? would it be that it's a smaller kernel, and maybe the grist isn't getting smashed enough??