Along with lower taxes and financial deregulation, higher interest rates were supposed to give a big boost to US banks after Donald Trump's election.
Executives have been telling investors they will be able to push up charges for borrowers, pay a pittance to savers and pocket the difference.
The Federal Reserve has obliged by increasing base rates three times since December.
However, a mixed batch of financial results in recent days shows that a recovery for banks from years of depressed returns is not as straightforward as some investors believed.
"Interest rates are going up, and we're starting to see some benefits, but in some cases it's not quite turning out to be the case," said Chris Wheeler, US bank analyst at Atlantic Equities. "We're still trying to understand what the full impact is."
The dynamics in the US hold lessons for executives in banks based in Europe and elsewhere, who are similarly hoping tighter monetary policy could give succour to their institutions.
Bank of America, Citigroup and JPMorgan Chase all posted lower net interest margins — the spread between what banks pay for their funds and what they earn from lending and other activities — in the second quarter compared with the first.
Among the big four US retail banks, only Wells Fargo managed to eke out an increase in the closely watched metric.
Sadly for bank investors, the interest rate set by the Fed is far from being the one that determines bank profitability. Yields on bonds, which are set not in Washington but in the capital markets, are just as influential. These have tumbled since March as investors have taken a dimmer view about the prospects for the US economy and inflation.