Imperial stout fermentation

Last Wednesday I brewed an imperial stout partial mash recipe. My OG was 1.090 which is right about where I wanted it. I pitched 2 packets of Irish Ale Wyeast 1728. Fermentation took off like crazy but only a couple millimeters of krausen formed after a couple days. After 4 days there are no visible signs of fermentation so I took a gravity reading 1.050 nowhere near my target. One of the smack packs never swole and was dated Jan 12 2012, it's been between 65 and 68 degrees, and during the mash I added too much heat for around 10-15 minutes at 160. I'm thinking the pack that didn't swell was bad, I screwed up the starch conversion with the high temp, or I'm being way to impatient. Planning on taking another gravity reading on Monday. If its stuck at 1.050 what should I do?

If the problem really is related to a bad yeast pack, go buy another pack and pitch it. Should still bring the gravity down (If that's really what the problem is). It's not like a bad yeast packet will cause the sugars to lose fermentability.

So its only been 4 days? 1728 hasn't been the fastest yeast for me but it always finished well and those temps look fine. 40 points in four days seems perfectly reasonable. Give it a week or two and take another reading.

Haputanlas said:

If the problem really is related to a bad yeast pack, go buy another pack and pitch it. Should still bring the gravity down (If that's really what the problem is). It's not like a bad yeast packet will cause the sugars to lose fermentability.

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Have you ever pitched yeast after primary fermentation is over? Wouldn't there be a lack of nutrients and oxygen for the new yeast?

Possibly. However, it's helped me in the past. I'd be too nervous about too much oxygen to shake more. Maybe a little bit of a rattle to the carboy/container to give a little bit of O2. But I'm sure it would be fine.