I'm on a 7th generation of WLP833. It had appeared to drop off in attenuation for at least one batch and has now jumped right back up. I suspect there are a few of things going on in my case, maybe yours too: One, I have had different recipes -- most were all base malt, the low one had a little honey malt, and the last high attenuator had some corn. Can't explain it all. Two, no matter how consistent I try to be, mashing in my homebrew system is bound to allow for slight variations in time and temperature patterns within the mash. Still can't explain it all. Finally -- the big one -- I'm not running a lab. There are bound to be vagaries in measurement, and actually, though the percentage variation I've recorded in attenuation looks significant, the actual variation in °P or gravity points is probably within the margin of accuracy and resolution of my tools and calculators. So, putting all those factors together and considering the aggrgate effects, in the end my conclusion is that there may actually be nothing to see here. So I'm telling myself: the fermentations are proceeding identically and normally in all other ways, the beer tastes damn good, and I just need to RDWHAHB. One apparent outlier doesn't mean anything.