When it began selling Big Macs for 55 cents two weeks ago, McDonald's Corp. was hoping the novel promotion would revive its U.S. sales growth. So far it hasn't.

Every day since it reduced the price of its flagship burger 75% to 55 cents, McDonald has surveyed more than 3,000 of its domestic restaurants, about a quarter of the total. Conclusion: Sales actually fell by as much as 6% compared with a year earlier.

Certainly,...