[euge]

You don't need to make a starter with dry yeast. Properly treated it (one pack) has enough cells to take care of a beer up to the upper .050's! And if you need more then another pack is easy enough to open.

I've never had any sort of attenuation problems with S-04. Were your "double batches" from the same wort or did you do separate boilings? Extract? Or did you mash? You haven't really supplied us with very much info about your recipe or methods so it is hard to diagnose a fermentation problem over the web.

[nateo]

Under "troubleshooting low attenuation" (p.274-5) Chris White says this:

"Add more yeast. Many brewers ask if they can just toss in some dried Champagne yeast to finish out fermentation. Those who say it works were probably dealing with a beer that had large amounts of simple sugars remaining, as Champagne yeast will not consume the longer wort sugars."

So, I actually haven't read the book in its entirety, but it looks like we came up with the same conclusion re: Champagne yeast.

[Joe Sr.]

Regardless of the benefits (or lack thereof) of Champagne yeast, I just plain don't like the flavor of beers where it has been used.  I've used it (long ago) a couple of times and did not find it to be as "neutral" as people claim.

If you don't have another s-04 yeast cake to use, I'd grow up a starter and pitch that.  I've had some luck doing this, but not as much as using a big ol' cake.

[denny]

Regardless of the benefits (or lack thereof) of Champagne yeast, I just plain don't like the flavor of beers where it has been used.  I've used it (long ago) a couple of times and did not find it to be as "neutral" as people claim.

Exactly the same conclusion I reached.

[mpietropaoli]

OK, so here is some background on the 2 brews:
 
For both, at flameout, I immersion-chilled to ~120 degrees, then dumped the entire kettle into a fermenter (including trub).  I let sit in my fermentation fridge with the temp control probe set to my pitching temp (in both cases here, 65 degrees), then dump the ‘cooling’ fermenter contents vigorously into a clean, sanitized fermenter, leaving the settled trub behind, aerate with sanitized mash paddle for 2-3 min, and pitch yeast.  This gives me a clean yeast cake on the bottom after floc and shortens my brew day.  Have done three different brews this way and had vigorous fermentations. 
 
As I said, for these two brews, I only had one packet of S-04, so a few days before brewing, I made a 1.5L starter, which took off after 5ish hours on the stirplate.  Then I set the starter in the fridge, to be decanted prior to pitching.  The yeast was probably about 45 degrees when I pitched into the 65 degree wort (both batches).
 
Pitched about 0.8 cups of pure decanted slurry into each fermenter about 12 hours after brewing, after vigorous pouring/splashing for aeration.  Since I have one fridge and one temp controller, I set the temp controller to a few degrees below my desired fermentation temp, in this case 61 degrees and 65 degrees, respectively.  The probe was suspended in the chest freezer between the two fermenters.
 
For both brews, it was a 75-minute brew-in-a-bag mash with a single sparge.  Roughly 76% mash efficiency on both.
 
Grain bill for IPA #1 (the one that is stuck @ 1.028ish)
80% Maris Otter
10% Vienna
10% Munich
60 minute boil, magnum hop charge @ 60, various late hops
5.5 gallons in fermenter at 1.061
 
Grain bill for IPA#2 (the one that got down to about 1.018)
80% American 2-row
10% biscuit
10% Munich
60 minute boil, magnum hop charge @ 60, various late hops
5.5 gallons in fermenter at 1.062
 
The krausens on both beers dropped after about 2 days, after which I roused the yeast by swirling the fermenters, then increased the chamber temp to 66 degrees.

The big possibility I am thinking about is that the chamber got too cold and one fermentation continued while the other yeast went to sleep.

[mpietropaoli]

Sorry, also mash temp of 152, roughly 2Q/lb mash thickness. 

[nateo]

IMO it's better to have fewer active yeasts than more inactive yeasts. I'd bet chilling and decanting was your problem. If you properly rehydrate dry yeast, you'll get ~90%+ viability, and 20b cells per gram. Proper temp is in the 100-105*F range, but that varies a bit by strain. IIRC viability will drop by ~10% with every ~10* drop in rehydration temp, but that might not be exactly linear.

It's partially a WAG, but I'd bet you'd have been better off just rehydrating and splitting the packet between the two.

[mpietropaoli]

IMO it's better to have fewer active yeasts than more inactive yeasts. I'd bet chilling and decanting was your problem. If you properly rehydrate dry yeast, you'll get ~90%+ viability, and 20b cells per gram. Proper temp is in the 100-105*F range, but that varies a bit by strain. IIRC viability will drop by ~10% with every ~10* drop in rehydration temp, but that might not be exactly linear.

It's partially a WAG, but I'd bet you'd have been better off just rehydrating and splitting the packet between the two.

Ok, I hear ya, but I pitched almost a cup of slurry!  Most say that is borderline over pitching!  It's just weird.