An under-attenuated beer can be problematic for a couple of reasons. The main reason is that the taste will be sweeter than you may want considering that you hopped with an expectation of a drier beer and the mouthfeel fuller as a result of the remaining sugars in solution. The second problem effects those that are bottling the beer. Normally you would add approximately half a cup of bottling sugar for a five gallon batch (48-12oz. bottles) to achieve the carbonation that you want in the finished beer but with all that residual sugar in solution to begin with, you may just be creating bottle bombs caused by the over carbonation of the additional sugar.
In the case of this beer, my expectation was for a final gravity of around 1.012 and so I am a little dissapointed but determined to get the rest of those points out of it one way or another. My belief is that the temperature got too cold and I may not have oxygenated the wort well enough causing the yeast to drop out of solution assuming a dormant state. My solution?
1.)pitch a fresh pack of US05 ale yeast
2.)gently stir up the yeast cake with a sanitized spoon to get it back into solution
3.)warm the beer up to 75f. with a 60watt light bulb placed strategically near the fermenter
Not real dignified but I have high hopes for success here, I have to.
In the past, US05 has been successful in getting a couple more points out of the gravity and I've read of others using Champane yeast with good results. The other part of this equation is patience. It may take an extra week or even two for the yeast to slowly do their job. I have to remind myself of this regularly as I tend to want the beer done quickly and into a keg or bottle to prevent any additional exposure to the elements since I basically 'open ferment'.
Finally, if the beer doesn't ferment any further after all these efforts then I will reduce the amount of bottling sugar I use. Additionally, I will plan to increase the amount of hibiscus flower that I add because it causes a distinct sense of dryness to the mouthfeel in this recipe.
If you've got more solutions to this common problem, please leave a comment.