Cause 1: Too Cool This situation is commonly referred to as a "stuck fermentation" and can have a couple causes. The simplest cause and probably the most common is temperature. As previously discussed, a significant drop in temperature can cause the yeast to go dormant and settle to the bottom.
Cure: Moving the fermentor to a warmer room and swirling the fermentor to stir up the yeast and get them back into suspension will often fix the problem.
Cause 2: Yeast The other most common cause is weak yeast. Referring back to previous discussions of yeast preparation, weak yeast or low volumes of healthy yeast will often not be up to the task of fermenting a high gravity wort. This problem is most common with higher gravity beers, OGs greater than 1.048.
Cure: Add more yeast.
Cause 3: Low Attenuating Extracts Another common cause for extract kit brewers is the use of extracts high in dextrins. Two brands are known to be high in unfermentables, Laaglanders Dry Malt Extract (Netherlands) and John Bull Liquid Malt Extract (UK). These are not bad extracts, in fact they are high quality, but their use is better suited to heavier bodied beers like strong ales, porters and stouts, where a high finishing gravity is desired.
