Contouring accurately can be quite difficult, especially for certain organs. In rare cases, you may even score a "zero" in terms of your StructSure score. For example, we found that the vast majority of people score a zero on the Brachial Plexus on their first try!
A very low or zero StructSure score does not mean you missed the organ entirely. To see how much you overlapped the gold structure, just look at the Dice Coefficient (DC). If the DC is above zero, it means you contoured part of the organ correctly. As the DC gets higher (and approaches a max of 1.00), it means your contours overlapped the gold contours more and more.
The StructSure score, however, is far more "sensitive"; this means it catches all errors and penalizes based on the degree of the error in terms of how-far-away your contoured structure was from the gold, for every errant volume element. The StructSure score has proven to be useful in estimating potential dosimetric impact in modern radiation treatment planning (Nelms et al., see References below).
So, don’t feel bad if you get a very low StructSure score on a difficult organ. Just inspect the differences, study the learning material and watch the walkthrough video, then redo and practice until you are able to limit the differences.