NEW YORK (CNN/Money) - Oops, it happened again.

Gap Inc.'s glitzy MTV-inspired fall ads featuring actress Sarah Jessica Parker and musician Lenny Kravitz may be hip and fun to watch, but has the combined star power of Parker and Kravitz successfully lured shoppers in droves into the Gap?

Not quite.

Sales in September at the company's namesake stores open at least a year -- a key retail measure known as same-store sales -- were down one percent compared to a much more robust performance a year ago. Overall same-store sales last month for all three of its retail chains -- Gap, Old Navy and Banana Republic -- declined three percent year-over-year.

This is the second year in a row that a big-name celebrity campaign for the Gap brand hasn't led to hefty gains at the till.

A similar thing happened last year when the San Francisco-based No. 1 apparel chain enlisted pop diva Madonna and hip-hop artist Missy Elliott to jazz up back-to-school sales. The ad featured the two improbable Gap-shopping "friends" pitching the retailer's corduroy pants and jeans.

At the time, the retailer said consumers hadn't taken to the jeans as well as it would have hoped.

Analysts say blaming the fickle consumer for their shortfalls always is a handy excuse for retailers.

Actress Sarah Jessica Parker and singer Lenny Kravitz debuted in Gap's fall ad campaign this year.

"We thought the Parker-Kravitz ads were catchy but the product lacks depth," said Harry Ikenson, analyst with First Albany Corp. Ikenson argued that Gap's fall collection is nothing more unique than basic jeans, tops, sweaters and blazers.

"In the ad, Parker wears a brooch on her sweater to make it look different. The problem with that is that other retailers in the mall are doing the same thing," said Ikenson. "That's not really a differentiated product. We looked at the stores, both at Gap and Old Navy, and found that there simply isn't enough newness on offer."

Ikenson recently downgraded his rating on Gap to "neutral" from "buy," saying that he expects the sluggish sales trend to continue into next year.

After an impressive 20-consecutive month run of positive sales growth spearheaded by CEO Paul Pressler, Gap's recent sales misses have sparked some concern among industry watchers who say the retailer is under pressure to prove it still has momentum to keep growing.

In fact, the retailer has posted monthly same-store sales declines for every month since June this year.

Getting the fashion right is only part of the story.

"Gap was testing itself against much earlier comparisons when it was in the midst of its two-year turnaround. The numbers have gotten tougher since then," said Mark Montagna, analyst with Wells Fargo Securities.

At the same time, Montagna said Gap's September sales performance was a disappointment because shoppers weren't baited by the heavy promotions during the period.

"We incorrectly expected the increased television ad budget for the Gap division to drive a low single-digit comparable sales increase," Montagna said. "Store traffic actually declined by two percent and the average unit retail price also declined. This tells us that the company had to take higher-than-planned markdowns in order to boost sales."

Some observers worry about deep discounts ahead of the key holiday sales period of November and December.

Bank of America Securities analyst Dana Cohen wrote in a recent research note to clients that 40-percent-off sales could cut into margins. Cohen, who has a "neutral" rating on Gap, reduced her 2004 and 2005 earnings estimates on the company.

Wrote Cohen, "We continue to have a very high regard for what [Gap's] management team has accomplished but we think at this point in the turnaround the risks are becoming more balanced....it seems that given the second-quarter trends, sluggish August and September promotions the easy turnaround period is behind them."

So where does Gap go from here?

"Gap isn't a growth story anymore. Its new fourth store concept targeting the 35-and-older customer will give it some growth," said Morningstar analyst Joseph Beaulieu. "But essentially the important thing for the retailer to do now is to stay the course, fix the fashion issues, make operational improvements, close underperforming stores, manage inventory and generate cash."

Investors have cooled to the Gap this year, with the stock trading down 25 percent from its 52-week high. Trading at a more svelte price-to-earnings ratio of 13.2 times next year's earnings, analysts say the stock looks cheap compared to its sector's P/E of 19.5, making it an attractive pick for long-term investors.  

--Analysts quoted in the story do not personally own shares of Gap Inc. and their firms do not have an investment banking relationship with the company.