Shoppers increasingly are ditching their desktops and doing their buying on mobile phones. But many small businesses aren’t making it easy for them.

Of the 64% of small U.S. businesses that have websites, says RBC Capital Markets, only 33% are optimized for mobile viewing. The result? It can be like having a “closed” sign on the door, says John Scheer, founding partner of Herman-Scheer, a creative design agency in Santa Monica, Calif., that works with many small companies.

“More...