There's never a shortage of losers in the stock market. Let's take a closer look at five of this past week's biggest sinkers.

Company

Feb. 21

Weekly Loss

Conn's (CONN 3.24%)

$33.16

38%

Groupon (GRPN 0.57%)

$8.03

24%

Financial Engines (FNGN)

$55.35

12%

GrafTech (GTI.DL)

$9.51

11%

Potbelly (PBPB -2.03%)

$20.11

11%

Source: Barron's.

Let's start with Conn's. The consumer-electronics retailer was Nasdaq's biggest loser last week, shedding 38% of its value after posting problematic preliminary quarterly results. Sales were strong, with an encouraging 33.4% spike in comps. However, adjusted earnings will clock in between $0.75 a share and $0.80 a share. Analysts were holding out for $0.93 a share.

The more alarming nugget in Conn's report is that the reason for the shortfall is an increase in its credit segment provision for bad debts. The percentage of outstanding consumer financing that's delinquent for more than 60 days has ballooned up to 8.8%. Conn's is selling. It's just having a problem collecting on the big-ticket items that it offers in-house financing on.

Groupon shares had a big reversal after initially spiking higher on its results for the holiday quarter. Revenue and earnings were better than expected for the period, but the daily-deals leader is bracing investors for a small loss in the current quarter. The market wasn't expecting that.

Financial Engines powered down on Friday after a rough quarter. Revenue and earnings barely missed Wall Street's targets, and the provider of defined-contribution managed accounts didn't do itself any favors by forecasting revenue in 2014 to grow by no more than $279 million. The pros were holding out for $287.7 million on the top line. Assets under management have grown to $88.2 billion, but the market doesn't like revenue growth decelerating to the mid-teens after climbing 29% in 2013. 

GrafTech slumped in sympathy with a rival's gloomy outlook. SGL Carbon warned that 2014 is off to a bad start with its graphite electrodes business. It sees operating earnings declining on the year and moved to suspend its dividend. GrafTech is also a provider of graphite material solutions, and with quarterly results coming on Thursday, investors wanted to steer clear of the company in case it also offers up an uninspiring snapshot of its niche. 

Finally we have Potbelly hitting a new low, after the sandwich maker didn't make enough dough. Sales in its latest quarter fell short of expectations, held back by a mere 0.7% increase in comps. There have been several eateries checking in with unimpressive store-level growth, but Potbelly went public last year on the thesis that it was in a fast-casual niche that was thriving as fast food and casual dining struggled. Something's not right if comps can't keep up with inflation.