Yahoo! (YHOO) CEO Marissa Mayer has been at the helm for more than three years. And during much of that time, investors having been rewarded due to the increasingly valuable Alibaba (BABA) - Get Report stake.

Yet that narrative changed over the past 12 months, as investors look past the Alibaba ownership and towards an increasingly stagnant core business. That has caused some investors to wonder when the jig is up.

Sunnyvale, Calif.-based Yahoo!, slated to report third-quarter earnings on Wednesday after the bell, has had a rough 2015. Shares have fallen 13%, compared to a slight loss in the S&P 500, as investors worry about the core search and advertising business.

"Growth has not been impressive over the last few quarters, and the onus will be on management to prove that their turnaround plan is actually working," Barclays Capital analyst Paul Vogel said in a research note. "Until we see evidence of that, we remain on the sidelines."

"MaVeNS" is at the center of Yahoo!'s product strategy and focuses on mobile, video, native advertising and social. While Mayer and her team have talked about the impressive growth of this group -- up 55% year over year to $255 million in the second quarter -- investors have noted there is confusion about this. SunTrust analyst Bob Peck recently cited how an ad on the Tumblr app can count toward both mobile and social revenue.

Investors have become frustrated that despite all of the talk about mobile, approximately 80% of the company's total revenue still comes from its desktop advertising business. By comparison, Facebook (FB) - Get Report generated 76% of its advertising revenue from mobile in the second quarter.

Yahoo! earned an adjusted 16 cents a share on $1.243 billion in revenue, including traffic acquisition costs, in the second quarter, up from $1.084 billion in the second quarter of 2014. If you strip out those traffic acquisition costs, Yahoo! generated $1.043 billion in revenue this year, compared to $1.04 billion in the year-ago quarter. That's a lot of work to get to exactly the same spot.

Analysts surveyed by Thomson Reuters expect the company to earn 17 cents a share on $1.255 billion in revenue for the third quarter.

"Despite early success with the MVNS initiatives and a long honeymoon period with the CEO, overall Yahoo! remains challenged to keep up with growth in an industry that's innovating faster," Cantor Fitzgerald analyst Youssef Squali said in a note to clients. "The stock remains cheap on a SOP [sum-of-the-parts] basis, but the primary catalyst remains the BABA spin off and its tax treatment." Squali rates shares as a buy with a $45 price target.

While the MaVeNS and the rest of Yahoo!'s core business will certainly not be ignored, additional attention will be paid to the company's spinoff of its stake in Alibaba and the Yahoo! Small Business unit, which will become a new company, Aabaco Holdings. Despite potential issues with the Internal Revenue Service, Yahoo! has said previously it expects to complete the spinoff in its fiscal fourth quarter and that it will go forward with the spin.

Analysts are particularly interested in whether the spinoff, which could be worth billions, will come with potential tax implications that might have to be shouldered by shareholders.

Bank of America Merrill Lynch analyst Justin Post believes shares are not discounting any tax savings, making shares potentially attractive. Post rates Yahoo! shares a buy with a $48 price target.

The company has also had to deal with an accelerated loss of senior leadership, most recently with Chief Development Officer Jackie Reses leaving for payments companySquare. In a recent research note, SunTrust analyst Bob Peck said, "Recent acceleration of losses is a focus of concern for investors."

Aside from Reses, Yahoo! has also lost Senior Vice President of Marketing Partnerships Lisa Licht, Head of European Operations Dawn Airey, Chief Marketing Officer Kathy Savitt, Vice President Eric Lange and others, all since the start of the third quarter. (Re/code's Kara Swisher has a complete look at executives who've left the company.)

Talent losses, a stagnant core business and the increased concern about ad blocking have all added up to an enormous headache for Mayer and Yahoo! Three years in, with the honeymoon period long over, investors are increasingly asking whether Mayer indeed is the right person for the job. Or is the situation so bleak that no one can turn around Yahoo!?