Of course, “under-achieving” is a relative term, isn’t it. You might actually be achieving results commensurate with your real abilities, rather than the abilities you think you have.
And laying aside external factors like bad luck, lack of money, horrible bosses etc we have (drum roll please)
- Fear (of failure/success/hard work)
- Resentment (why should I do x when a) other people are free-loading or b) “other people caused this mess, not me – it’s not fair/they should be the ones sorting it out”)
- Lack of focus