The above picture demonstrates what shadows should look like. Flushed against the ground or wall, and rendered at a depth appropriate for the scene.
Leading problems with shadows include:
Shadows are missing altogether when they should be active.
Shadows create a dark film in one or both eyes.
Shadows are offset in one eye. They seem disconnected from the object they represent.
The shadow is rendered at screen depth or in 2D compared to the rest of the scene. The shadow may seem to float off the ground or wall.