Son and I had terrible efficiency in the first of two batches (DNERIPA) brewed earlier this week with the second (NB Brut DryPA) batch dead on. The DNERIPA was supposed to end up about 1.100 per BeerSmith but ended up about 1.080.
We've had problems in the past and corrected them with tweaks to our process and have run 75-80% efficiency since. We've never adjust water chemistry (on a great well). Milled the grain ourselves and it looked fine. Volumes looked good. Checked temp at mash-in and at the end and we lost 3< degrees. I've thought about the day and am at a loss. I'll probably cue this exact recipe up again and see what happens.
What do you look for in hindsight when it comes to efficiency problems? Do you ever have one-off "bad runs"?