1. Lack of motivation. A talent is irrelevant if a person is not motivated to use it. Motivation may be external (for example, social approval) or internal (satisfaction from a job well-done, for instance). External sources tend to be transient, while internal sources tend to produce more consistent performance.
2. Lack of impulse control. Habitual impulsiveness gets in the way of optimal performance. Some people do not bring their full intellectual resources to bear on a problem but go with the first solution that pops into their heads.
3. Lack of perserverance and perseveration. Some people give up too easily, while others are unable to stop even when the quest will clearly be fruitless.
Read the rest here: Why Intelligent People Fail...