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](http://www.merriam-webster.com/dictionary/underachiever)

>- "a student who does not perform as well as expected or as well as the IQ indicates" [TFD](http://www.thefreedictionary.com/underachiever)

>- "a person or thing that performs below expectations." [D.com](http://dictionary.reference.com/browse/underachiever)

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.