The short and so far disastrous public run for Potbelly (PBPB) was continuing Wednesday, as shares of the sandwich seller slumped following its second earnings report since its market debut last fall.

In recent trading, Potbelly was dropping more than 10% to to $20.10, mere pennies above the lowest level it hit on the session. That meant it was achieving two inglorious milestones simultaneously — having the biggest single-day decline in its history and sliding to the worst price it's seen post-initial public offering.

As for Potbelly's numbers specifically, on an adjusted basis it earned 6 cents a share in the fourth quarter, 2 cents ahead of estimates carried on FactSet. Sales were another story. For the quarter, the top line ticked up 1.7% to $74.8 million — 9.9% excluding an extra 14th week in the prior year quarter — but fell short of expectations by about $1.2 million. Same-store sales at company-owned shops edged ahead 0.7%, if the extra week in 2012 is taken out. In all of last year, 42 new stores opened, including 34 of the company-operated variety. Total revenue rose 9% to $299.7 million, and comp sales were up 1.5%.

Potbelly's report wasn't exactly a tale of woe. It just wasn't a tale of feats of greatness, either, and therein lies the trouble. Potbelly attributed the tepid showing in part to bad weather, saying there was "no question the external environment was disruptive." This is a well-worn excuse that's been used numerous times by retailers during this particularly snowy season. But Potbelly has bigger issues than wintry weather.

Wall Street appears to be of the mind that competition is making it hard for Chicago-based Potbelly to stand out. Setting aside the fact that there are a host of restaurants in any American town, the company has to try and differentiate itself in a segment that has competitors including Subway, Jimmy John's, Firehouse Subs, Jersey Mike's, Quiznos, Schlotzsky's, Panera (PNRA), McAlister's Deli and Jason's Deli. Outside of chains, there are local delicatessens. You could even simply make a sandwich at home.

Clearly, there's no shortage of options, so a customer looking to pay in the neighborhood of $10 and up for a sandwich, soup and soda at lunchtime won't necessarily choose the Potbelly brand unless that's their personal preference. You can pick your toppings, which plays into the "customized" food mania sweeping the food arena, though that's hardly innovative considering any number of sandwich makers do the same.

The drop in Potbelly suggests the restaurant industry may not have another year in which simply being a member of the group translates to big gains. In 2013, a set of 38 restaurants rose almost 55% and nearly doubled the S&P 500's return, extending a strong, positive stretch that dates back to 2009, the start of the bull market. Since the new year began, however, a good number of those high fliers have surrendered a bit of that ground.