Big Tobacco is finally making its big push into electronic cigarettes. Altria Group Inc. and Reynolds American Inc. together have captured about a quarter of convenience-store sales just weeks into national rollouts of the battery-powered devices.

In a surprising development, though, e-cigarette sales are falling at these traditional retail outlets, reversing three years of rapid-fire growth. That raises questions whether Altria and Reynolds, which already dominate the U.S. tobacco market with a combined 75% market share,...