Shares in Google have dropped 5% despite the technology giant reporting a first-quarter profit rise of 3%.
Profits were $3.45bn (£2.05bn), but investors are preoccupied by Google's inability to maintain advertising prices.
A widely watched measure, the average "cost per click", was down 9% from a year earlier.
Another weak spot highlighted in the report was the firm's discounted sale of Motorola Mobility to Lenovo.
Google sold the smartphone maker to Lenovo in January for close to $3bn, after paying $12.5bn for the firm less than two years ago.
Despite investors' reaction, Google's chief executive, Larry Page, was upbeat: "We completed another great quarter," he said in a statement.
"We got lots of product improvements done, especially on mobile. I'm also excited with progress on our emerging businesses."
However, Google continues to struggle with its ability to charge advertisers higher prices for mobile ads, which are increasingly important with more and more consumers accessing Google's browser through their smartphones.
Advertisers have proven reluctant to pay as much for ads on mobile screens compared to Google's bread-and-butter desktop ads, which have been the main revenue generator at the firm.