you would expect their offspring to be of similarly above-average intelligence

Are you referring to the fact that second generation immigrants are not as successful as their first generation counterparts despite being the offspring of first gen immigrants? I think there's a correlation between success and intelligence, but I don't think it's a guarantee; lots of "intelligent" people are not successful and not all successful people are "intelligent". Also, human offsprings are a mix of their parents. Things like crossing over during meiosis create new gene combinations. Basically, there's an element of randomness and smart parents don't always have smart kids (and vice versa). I would further argue that one of the bigger aspects of success (financial/career) is determination and the will to succeed. So while intelligence is somewhat hereditary and success is loosely correlated with intelligence, it's not going to be as direct as if you picked out and interviewed the people yourself from a pool of eager applicants.