At a minimum we need the OG, FG and the yeast used to best determine if the yeast did what it was expected to do. Subtract the OG from the FG (for ex, 1.055-1.015 = 40) and divide 40/55 and that is the attenuation achieved. Here it would be 72%. Then compare that to the yeast's typical AA%.
Significant differences could be caused by numerous things, so the more detail provided the quicker potential causes can be eliminated, leaving a most-likely culprit.
Old package, no starter, too cold, temps fluctuating, high ratio of crystal grains, moon phases, etc.