Here's the deal:
I mash for 1 hour, no mashout, I use 5.2 stabilizer, I batch sparge in 2 equal amounts and I consistently average 85% lauter/sparge efficiency.
Today I brewed a Dubbel and I only got 68% efficiency. I used a malt, Franco-Belges Pilsen, that I have never used before as my base malt, and I didn't preheat my mash tun well enough because my mash temp went from 152 at the start all the way down to 145 at the end. It normally only drops 3 degrees or so.
Speculate with me fellas - what could be the culprit? The Franco-Belges Pilsen, drop in mash temp., a combination of both? I don't think the drop in mash temp should even affect efficiency, just fermentability of the wort, but I'm not an expert so I can't be 100% sure.