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
"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 student who does not perform as well as expected or as well as the IQ indicates" TFD
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.