BlackBerry (Nasdaq: BBRY) shares tumbled for a second straight day Monday after Goldman Sachs Group Inc. cut its rating on the struggling smartphone maker, citing a disappointing debut for the company’s Z10 phone in the U.S.

The shares fell 4.6 percent to $14.23 in New York, following a decline of 7.7 percent on March 22, the last trading day.

Before the two-day drop, the stock had climbed 36 percent this year, lifted by optimism that the company’s new lineup would fuel a turnaround.

The Z10 went on sale last week in the U.S. — BlackBerry’s biggest single market — almost two months after its rollout in the U.K. and Canada. The country is “critical for BlackBerry’s ultimate success,” said Goldman Sachs analyst Simona Jankowski, who found that initial sales at AT&T Inc. and Best Buy Co. stores were “tepid.”

“Our retail checks at over 20 store locations since March 22, including at AT&T, Best Buy, and RadioShack, revealed a surprising lack of marketing support and poor positioning of the product,” she said in a report today. “We also saw limited advertising around the launch.”

Jankowski cut her rating on the stock to a neutral and now gives the new BlackBerry lineup a 20 percent chance of being successful, down from 30 percent.Investors are keeping a wary eye on BlackBerry (Nasdaq: BBRY) as markets open in the U.S. Monday after the launch of the company’s new smartphone failed to spark much interest on Friday.

[The company operates a research-and-development office in the Research Triangle.]