Closing at all-time highs on back-to-back days, the Dow Jones Industrial Average just seems to be showing off. The S&P 500 Index (SNPINDEX:^GSPC), though, isn't much more humble; it's only 1.5% below its own all-time highs. But no matter how hot the market is, there will always be at least a handful of laggards that can't seem to get things together. The three companies below show that just because you're in the S&P 500 doesn't mean you're always on top of your game.

Staples (NASDAQ:SPLS), up 17% this year before today's stumble, swiftly fell 7.2% to lead all laggards in the index today. The catalyst was simply a bleak earnings report -- one so bleak that the CEO himself characterized the current state of things as "a challenging sales environment." Well, it certainly wasn't an easy sales environment, as same-store sales in North America and Europe declined amid a tenuous global economy and competition from online rivals. 

This next stock has incessantly tried my patience and seems to take some sort of sick joy in mocking me. Yet again, for a third consecutive day, retailer J.C. Penney (NYSE:JCP) ranks as either the worst or second-worst stock in the 500-component index, falling 3.5%. Needless to say, that's a legendary track record of short-term underperformance. Catching on to the nearly hopeless position J.C. Penney finds itself in, both Citigroup and Oppenheimer downgraded the stock, citing rapidly decreasing sales that may take a while to correct. 

Lastly, online auction site eBay (NASDAQ:EBAY) dropped 3.1% on a comment from R.W. Baird analyst Colin Sebastian, who, despite maintaining an outperform rating, indicated that February sales grew at a slower rate than January's. The online payment market is brutally competitive, and as the mobile payment market becomes a more and more valuable battlefield, eBay will need to establish itself as a player there both for future success and for decent growth rates.