Cause 1: Lack of Preparation As I stated in the section above, yeast that are improperly prepared, whether from lack of re-hydration, lack of numbers (i.e. lack of Starter), or lack of aeration, will often fail to finish the job.
Cure: Pitch new yeast.
Cause 2: Too Cold Temperature can also be a major factor for fermentation performance. If the temperature of the room where the fermentor is cools down, even only 5 ¡F overnight, then the yeast can be slowed dramatically.
Cure: Always strive to keep the fermentation temperature constant, the yeast will thank you for it.
Cause 3: Too Warm The flip side of the coin could be that the temperature was warm, e.g. 75¡F, and the yeast got the job done ahead of schedule. This often happens when a lot of yeast is pitched, the primary fermentation can be complete within 48 hours. This is not necessarily a good thing, as ferments above 70¡F tend to produce a lot of esters and phenolics that just don't taste right. The beer will still be good, just not as good as it could have been. It will depend on your tastes and the yeast strain.
Cure: Always strive to keep the fermentation temperature within the recommended range, the yeast will thank you for it.