Shear locking is an error that occurs in finite element analysis due to the linear nature of quadrilateral elements. The linear elements do not accurately model the curvature present in the actual material under bending, and a shear stress is introduced. The additional shear stress in the element (which does not occur in the actual beam) causes the element to reach equilibrium with smaller displacements, i.e., it makes the element appear to be stiffer than it actually is and gives bending displacements smaller than they should be.
Long, slender beams in bending have greater curvature than do short, deep beams, and when modeled with quadrilateral elements, will be affected more by shear lock. Increasing the number of elements will allow a more accurate modeling of the curvature, and reduce the effects of shear lock.
For a full discussion, check in a finite elements text, or look at this link:
Mattman is right, also if you use a FEA package with a wide range of element types you can choose a redused integration element and this should sort the problem for you.