waste (one’s) potential

If a person is very intelligent or talented, you can say they have great "potential", which means they could be very successful. However, if they don’t do any work to become successful, it won’t happen. In this situation you can say the person is “wasting their potential”:

He’s wasting his potential just bartending, but it’s hard to make the leap to something new.

Another phrase that's similar is "live up to (one's) potential":

You have a lot of potential. That's why I'm tough on you: I want to make sure that you're living up to it.

This phrase appears in these lessons: