What can I do to ensure carbonation in the bottle?

[73standard]

I am about to bottle my imperial stout and I think it's going to be a difficult one to carbonate in the bottle. This beer clocks in at 12-13% ABV depending on which formula you use. I primed and bottled 2 bottles so far just as a test. After 1 week there was a hiss when the bottle was opened; I'd say it was about half has big as the hiss when you open a normal beer. It poured no head and seemed like 25% carbonated. The other bottle I tried after 2 weeks was similar.

What can I do to help this big boy carbonate when I bottle the rest of the batch?

Don't know if it matters but this was a malt extract recipe, fermentation got stuck at 1.08, and I got it unstuck with a starter of WLP099 and it ended up at 1.027ish. So there is WLP099 in there. This particular batch of yeast was pitched about 3 months ago in the effort to get it unstuck.

 

[flars]

You got a"hiss" after two weeks that is good. If your normal carbonation schedule for an average gravity beer is two weeks of conditioning and one week of chilling to force carbonate, expect this large beer to take longer. Give it up to two months of conditioning and a week of chilling.

The high alcohol content will slow the yeast down quite a bit.