In the relative calm that is the market for U.S. Treasuries, a sense of unease over a vital cog in the financial system’s plumbing is beginning to rise.

The Federal Reserve’s bond purchases combined with demand from banks to meet tightened regulatory requirements is making it harder for traders to easily borrow and lend certain desired securities in the $1.6 trillion-a-day market for repurchase agreements. That’s causing such trades to go uncompleted at some of the highest rates since the financial crisis.