What: Shares of Del Frisco's Restaurant Group (NASDAQ:DFRG) slumped on Thursday after the company missed analyst estimates for both revenue and earnings when it reported its second-quarter results. At 2:30 Thursday afternoon, the stock was down about 16%.

So what: Del Frisco reported quarterly revenue of $73.8 million, up 9.5% year-over-year but about $1.5 million short of analyst estimates. This growth was driven by new restaurant openings, as total comparable-store sales declined by 1.4% during the quarter.

Comparable-store sales increased at only one of the company's three segments. At Del Frisco's Double Eagle Steak House, comparable-store sales rose 1%, driven by a 5.8% increase in average check offset by a 4.8% decrease in customer counts. The other two segments, Sullivan's Steakhouse and Del Frisco's Grille, saw comparable-store sales declines of 3% and 6.3%, respectively.

Del Frisco reported EPS of $0.16, down from $0.20 during the same period last year and $0.03 short of analyst estimates. The company lowered its guidance for comparable-store sales for the full fiscal year, now expecting an increase between 0.5% and 1.5%, down from a previous range of 2%-3%. Del Frisco expects revenue to increase by 10%-13%, driven by seven new stores being opened.

Now what: Investors are clearly disappointed in Del Frisco's results. While the company is growing revenue by opening new restaurants, its existing restaurants aren't performing well. In addition to declining comparable-store sales, restaurant-level EBITDA as a percentage of revenue declined by one percentage point year-over-year to 22.3%.

Del Frisco's stock has declined by about 50% since it peaked a little more than one year ago. The stock trades at about 20 times 2014 earnings, but with comparable-store sales growth expected to be anemic this year, and with profitability declining, the stock's decline may not be over.