Someone who's a nonstarter never seems to succeed or achieve their goals. A baseball player who strikes out every single time he's up at bat is a nonstarter.
A political candidate who loses every election is a nonstarter, and an entrepreneur whose businesses keep failing is also a nonstarter. You can also use this word to describe a plan or idea that's just never going to work: "I can see why you'd want to bring your pony to school, but I think it's a nonstarter." The original meaning of nonstarter was a horse unable to run in a race at the last minute.