The promise of low borrowing costs for longer just doesn't pack the punch it used to.

Last week should have been fantastic for the $1.3 trillion U.S. junk-bond market: The Federal Reserve scaled back its prediction for how quickly it will raise benchmark interest rates, while also expressing confidence in the world's biggest economy. That's almost an ideal world for junk bonds.

And yet investors yanked $1.3 billion from mutual funds that buy the debt last week, and they've pulled $2.9 billion this month, according to data compiled by Wells Fargo & Co. Dollar-denominated high-yield bonds, while rallying some immediately after the Fed statement was released Wednesday, have lost about 1 percent in March after gaining 2.4 percent the month before, Bank of America Merrill Lynch index data show.