As the population ages more people are retired. Typically retired people have lower incomes than they did during their working years. The growth in the number of elderly depresses median household income. Normalizing for the size of household radically changes the picture. Other factors further depress the apparent increase in household income.
Immigrants typically have lower incomes.
Increased immigration may reduce median household income in a way that does not necessarily reflect on the state of the economy. The Consumer Price Index used to normalize income itself has technical problems. In some cases, it does not appropriately consider the improvement in products. If a car increases in both price and reliability, not all of the increase is really inflation. The consumer in some sense is getting more car.