Shares of SmileDirectClub Inc. /zigman2/quotes/214109287/composite SDC -9.36% slid 1.1% in afternoon trading Tuesday, to buck a broader market rally, after Stifel Nicolaus analyst Jonathan Block abandoned his bullish stance on the teeth-alignment treatment company, saying there are "more headwinds than tailwinds" on the horizon. Block cut his rating to hold, after being at buy since initiating coverage of the company in October 2019, about a month after it went public. He also slashed his stock price target to $7 from $9. He said it appears SmileDirectClub is losing market share in the North America adult clear aligner market share to Align Technology Inc. /zigman2/quotes/200300692/composite ALGN -0.83% , and to other direct-to-consumer companies, such as Dentsply Sirona Inc. /zigman2/quotes/204544124/composite XRAY +2.61% . And Block said he has been unable to identify meaningful progress in the professional channel, which the company has identified as a future growth initiative. The stock has soared 36.9% since it closed at a 16-month low of $4.69 on Aug. 19, but was still down 29.8% over the past three months. In comparison, Align Technology shares have climbed 15.4% the past three months and the S&P 500 /zigman2/quotes/210599714/realtime SPX -0.84% has gained 3.2%.