As described on other pages, understanding human anatomy, expression, and proportion is complex and involded. There are many pitfalls in sculpting a portrait. On this page you will see many works that have various problems that may help you better understand what to look for in your own work so that you can hopefully avoid them. Some may simply be incomplete or unresolved, and others may have incorrect proportions, eyes that do not feel spherical, bone structure that is off, alignment problems between the left and right side of the face, mouths that do not curve along the jawline properly, or other issues. Many do not have resolved necks and torso proportions or details.
In creating a bust, common problems include not centering the eyes between the chin and top of the head. It is common to make the forehead too small. Many people don't inset the eyes into the skull or frame them within the eye socket. The eye socket sets back on the sides of the face. It is easy to miss that the jaw is a very round form, and the lips must contour around it. Often jaws are flattened, and the lips follow suit. The left and right termination of the lips should form depressions that give the mouth character and accuracy.
Examine these student examples and try to describe for yourself what is - and is not - working. It is much easier to see flaws in another person's work than in your own, but the more you can understand of others' problems the more you will be able to discern problems in your own work. When examining your own work with a critical eye, sometimes it simply takes stepping away from it for a time it to see it dispassionately.