July was a bit of an off month at BJ's Wholesale Club (NYSE:BJ), with only a slight increase in same-store sales. The previous two months' comps came in quite a bit stronger. It's not enough for me to give up on the company, but it may be time to put BJ's on a short leash. Let's take a closer look.

Comps rose just 1.5% in July. Part of the reason was gasoline sales, which subtracted 0.6% from the July total after having helped to boost the numbers in past months. BJ's generally prices gas below the average in a particular market, to help drive traffic into the stores, yet even traffic -- excluding gasoline -- decreased 1%. This dip, however, was caused in part by a change in its spring membership trial, which cut off this year on July 9. Last year, the trial ran until the end of July.

On the positive front, the average transaction at BJ's grew 4%, meaning customers spent more once they got inside the stores. Doesn't surprise me too much -- my wife can never leave the store spending less than $100, since, after all you could always use that jumbo bottle of detergent.

Continuing to be BJ's strong suit, food sales increased 5% on a comparable basis. Maintaining growth here is important, since food accounts for about 60% of BJ's general sales, and it shouldn't be too hard since, given rising prices, consumers are likely to continue buying food in bulk quantities to save some money.

But the company still has to get its customers to buy other higher-margin items. General-merchandise comps declined 1% on lower sales of air conditioners, furniture, and toys. Even though books were a strong category, it was driven by one item -- the new Harry Potter title.

BJ's has some significant advantages over rivals Costco (NASDAQ:COST) and Sam's Club, a division of Wal-Mart (NYSE:WMT): It accepts coupons, it sells certain items in smaller packages, and it accepts more credit card types than Sam's or Costco do. But even so, BJ's has not yet used these advantages to drive customer growth and spending on those higher-margin items. For my money, I'll ride the tide a little longer. But patience has its limits.

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Fool contributor Larry Rothman is happy to receive feedback, and he promises to read it when he's not being wrestled by his three children. Feel free to email him at rothmanviews@comcast.net. He doesn't have any positions in the companies mentioned.