Shares of streaming-device maker Roku have fallen 16% from its post-IPO high last week — and the pullback could be an indication that investors are worried it can’t compete long-term against its much bigger Silicon Valley rivals.

Roku shares popped 68% on its first day of trading on Sept. 28, and then climbed another 13% Friday. But the stock fell 11% on Monday and shares are trading down more than 5% Tuesday. [UPDATE: Roku closed down 11.7% on Tuesday, to $20.81 per share.]

The stock dropped this week as investors assessed Roku’s “weak new lineup that doesn’t differentiate them too much from Apple, Amazon or Google,” said Patrick Moorhead, founder of and principal analyst at Moor Insights & Strategy, a technology research and consulting firm. (Moorhead said he doesn’t own any shares in Roku.)

On Monday, Roku unveiled its fall line of products, including the new Roku Ultra set-top, priced at $100 and offering support for 4K and HDR streaming, and the entry-level $30 Roku Express device.

That said, Roku’s shares are still trading well above the $14-per-share IPO price. It’s possible that shares have declined once early IPO investors cashed out of the stock. Roku shares on Tuesday saw a surge in demand from short-sellers placing bets that the stock will decline.

Roku has had a first-mover advantage — it launched the first streaming device for Netflix, in 2008 — and has won a small but loyal following, reporting 15.1 million active user accounts as of June 30, 2017. However, Moorhead said, “their biggest challenge is to find a long-term sustainable advantage.”

The scrappy independent streaming-platform developer has been able to beat Goliaths in the tech biz. Roku had 37% share of all streaming devices owned by U.S. broadband households in the first quarter of 2017, according to research firm Parks Associates. That put it as the No. 1 brand in the product segment, ahead of Amazon Fire TV and Fire TV Stick (24%), Google’s Chromecast (18%) and Apple TV (15%).

The company isn’t profitable, but it pared losses in the first half of this year. Per its IPO registration filing, Roku generated $199.7 million in revenue during the first half of 2017, up 23% from $162.3 million during the same time last year. It reported a net loss of $24.2 million for the first six months of 2017, versus a net loss of $33.2 million in the comparable year-earlier period.

Roku CEO Anthony Wood, in an interview with Variety last week, said the company sees tremendous opportunity ahead — laying out the hyperbolic (if not to say unrealistic) goal of powering “every TV in the world.”

“Our business model is focused on growing active accounts, and then monetizing those active accounts through our platform business,” Wood said. “The way we grow active accounts is we sell streaming players, we license to TV companies and we license to operators.”