Wal-Mart (NYSE: WMT) shareholders must wait a little longer for the retail giant to turn the tide. Although the company says its attempts to jump-start U.S. sales are gaining traction, you won't find any of that progress in its quarterly financials.

First-quarter net income increased 3.8% to $3.4 billion, or $0.97 per share. Revenue increased 4.4%, to $103.4 billion. However, Wal-Mart's U.S. same-store sales figure, the metric we've all been focused on, fell 1.1%. This marks the eighth consecutive quarter in which Wal-Mart's U.S. comps have decreased.

If you're looking for a bright spot, Wal-Mart's international sales jumped 11.5%, to nearly $28 billion. The company described its international business as a "growth engine." That's good, since the U.S. engine appears to have stalled, and doesn't want to start up again.

As much as Wal-Mart's international growth is helpful, ongoing attempts to add fuel to that particular growth engine won't always work out as planned. For example, it's currently grappling with difficult terms related to purchasing South Africa's Massmart, and it might retreat from that market.

The U.S. has its share of economic difficulties right now, including commodity inflation, nosebleed gas prices, and other grim realities that require thinking twice before investing in grocers and other retail stalwarts. Wal-Mart's tendency toward deep discounting doesn’t help it compete very well domestically these days.

Wal-Mart's misery doesn't really have company, though. Back in March, warehouse discounter Costco (Nasdaq: COST) reported impressive quarterly results, with same-store sales rising 7%, or 4% excluding gasoline sales. More recently, Costco reported a 12% April comps increase  -- 7% excluding gas and foreign currency. Costco will report its latest quarterly results next week, so we'll be able to compare, but it's been holding its own much better than Wal-Mart in recent months.

Costco's a formidable rival, but it's hardly the only one vying for bargain-hunting consumers. Family Dollar (NYSE: FDO), Big Lots (NYSE: BIG), and Target (NYSE: TGT) are just a few other retailers that have made discounts part of their missions.

Furthermore, for some time now Wal-Mart's been warning that its core customers are living paycheck to paycheck more than ever. Apparently the company's most frequent shoppers simply aren't well-insulated to the skyrocketing U.S. cost of living right now. That doesn't help any of the other discount players, either.

You'd think economic hard times would make things easy for the retailer, but Wal-Mart's recent sad U.S. sales performance says otherwise. Investors should steer clear until it shows clear signs of revving up its U.S. sales engine once again.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.