What about "an underachiever"?
- Someone who always fails to reach their full potential. Often used in reference to students and athletes.
Definitions:
- "one that fails to attain a predicted level of achievement or does not do as well as expected" MW
- "a student who does not perform as well as expected or as well as the IQ indicates" TFD
- "a person or thing that performs below expectations." D.com
e.g.
- I wouldn't call your son dumb but take a look at his grades: he has been an underachiever, Mrs Taylor.
- When I had three questions put on hold on the same day, I felt like an underachiever.