Dollar Tree shares dropped more than 15% last week after the discount retailer reported lower-than-expected earnings amid flat sales for its Family Dollar unit.

For the second quarter, Dollar Tree’s net income rose to $274 million, or $1.15 per share, from $234 million, or 98 cents per share, in the year-ago period. Revenue increased 4.6% to $5.5 billion.

But Wall Street analysts were expecting earnings of $1.16 per share, contributing to a 15.5% plunge in Dollar Tree’s stock price on Thursday.

“Investors were conditioned to expect more, given recent stronger quarterly results from other retailers, like Wal-Mart Stores, Target and, perhaps especially, rival Dollar General,” Barron’s said.

Dollar Tree also lowered guidance for the fiscal year ending February 2019 to between $4.85 and $5.05 per share, from $4.80 to $5.10.

Dollar Tree’s same-store sales rose 3.7%, topping last year’s 3.9% gain and matching Dollar General’s latest quarter. It was the fifth consecutive quarter of same-store sales growth exceeding 3.5%.

But comparable sales were flat for Family Dollar, down from the 1% rise in the same quarter the year before. Analysts had expected an increase of 0.51%.

Poonam Goyal, a retail analyst with Bloomberg Intelligence, said it will take a while to fix the business. “Family Dollar has to play catch up with Dollar General,” which is much further along on initiatives to drive traffic, she said.

Excluding items, Dollar General earned $1.52 per share in its second quarter, beating analysts’ expectation of $1.49. “We delivered a strong second quarter and are proud of our team’s execution,” CEO Todd Vasos said in a news release.

“Our results this quarter were driven by contributions from our mature store base as well as the robust performance of our new stores,” he added.

Dollar Tree CEO Gary Philbin noted that the company opened its 15,000th store and 23rd U.S. distribution center in the latest quarter. “Both of these milestones call out the continued opportunities for growth across North America for our Dollar Tree and Family Dollar banners,” he said.