What economic problems lurked beneath the general prosperity?

I'm assuming you're referencing the time of the "Roaring 20s."

The apparent financial juggernaut of the stock market was a facade.  Most of the stock purchases were being done "on margin," meaning with borrowed money.  So the gains in the stock market were artificial and were setting up the country for a financial collapse and much suffering to come.
Also, under the surface image of prosperity at the top end of society, there was plenty of poverty underneath that.  Across America in the 1920s, more than half the country lived below the poverty line.  Black Americans were among the poorest in the country (as well as battling racism).  Farmers were in bad shape too.  A large number of Americans still depended on agriculture for their livelihood in the 1920s, and farming practices were not sound financially or agriculturally.  (The Dust Bowl of the early 1930s was a result of farming practices that caused widespread soil erosion.)