While the brand rolled out several new drinks during the quarter, including the Midnight Mint Mocha Frappuccino, an Ombre Pink Drink and the Iced Coconutmilk Mocha Macchiato, none elicited the same frenzy as the limited-time Unicorn.
"The slowdown included a lower-than-expected lift in non-discounted Frappuccino beverages following Happy Hour as well as somewhat lower-than-expected sales of other core beverages during the period," the company said.
And Johnson sees sales continuing to slow into the company's fourth quarter.
"We saw in the back half of last quarter, we saw industry data around away-from-home food and beverage that prompted us to take a cautionary posture for Q4," he said.
The company lowered its estimates for full-year 2017; it now expects to see earnings per share to be between $2.05 and $2.06. Previously, the company forecast EPS in the range of $2.08 to $2.12.
In addition, the company said it expects revenue growth will be on the lower end of its previous range of 8 percent to 10 percent.
"This is a near-term phenomenon," Johnson said. "Nothing to be concerned about long term."
Some may have expected Starbucks would benefit from back-to-school shoppers hitting the malls and discounters like Target, where Starbucks operates hundreds of cafes.
However, many retailers have been seeing softer sales in recent quarters. The National Retail Federation, the industry's trade group, expects fewer consumers will shop for back-to-school supplies in department stores this year.
That's bad news for Starbucks, since many of these department stores draw shoppers to the mall, where they might pick-up a Frappuccino while hunting for that first day of school outfit.
The NRF said that about 57 percent of customers will shop for back-to-school goods at department stores this year, down nearly 3 percent from last year.
In comparison, 46 percent of shoppers plan to purchase school items online this year. Last year the same percentage of customers said they would shop online, a 10 percent jump from 2015.
Shares of the company fell nearly 9 percent on Friday, amid several downgrades.