After some initial euphoria following Google's (GOOG 1.01%) (GOOGL 0.87%) earnings news last week, its stock has slowly but surely given up its gains. Other than recent rumors involving possible acquisitions including Salesforce.com and Twitter, there doesn't appear to be anything definitive creating the pressure on Google's share price.

That said, Google has some challenges to address the balance of this year, and beyond. Over the long run, the consensus analyst recommendation for Google as a "strong buy" is warranted as the company continues driving double-digit top line growth. But that doesn't mean there won't be a hurdle -- or three -- to overcome along the way. Here are a few possible hiccups Google will face moving forward.

Soaring overhead
According to Google CFO Patrick Pichette during last week's earnings announcement -- CEO Larry Page didn't participate in the call -- expenses jumped last quarter, and will continue to do so due to ongoing costs associated with infrastructure and facilities.

Add in Google's approximately 20% jump in headcount compared to Q1 2014, and it's no wonder operating expenses climbed over $1.1 billion to reach 37% of total revenue: up from last year's 35%. Ongoing investment is necessary to fuel growth for a company the size of Google, but as overhead continues to climb, investors will demand some results -- as they should.

Google's spending to enhance its mobile search ad tools is one example. It's no secret that mobile isn't Google's strong suit as yet. Improvements -- or lack thereof -- in this key area will be a good gauge of whether all that spending is paying off.

Speaking of mobile
Though small by comparison, there's a reason Facebook (META 0.60%) is increasingly cited as a threat to Google's digital advertising dominance: mobile. It's been a couple of years since CEO Mark Zuckerberg went all out to improve Facebook's mobile user experience, as well as enhance its advertising solutions. And it's working like a charm.

Not only are 1.25 billion of Facebook's 1.44 billion monthly average users, or MAUs, of the mobile variety, but 73% of its $3.32 billion in ad revenue last quarter came from mobile spots. And with each successive quarter, Facebook's mobile revenue as a percentage of its total sales is climbing. By contrast, last quarter's 13% decline in Google sites cost-per-click, or CPC, rates is largely due to mobile.

Google generates much of its ad revenue via search, something its marketing partners simply won't pay as much for on a smartphone. If you're buying Pichette's explanation, the reason for the quarter-after-quarter CPC declines is due to YouTube's "skippable" TruView ads. Apparently, TruView spots "monetize at a lower rate than clicks on Google.com." The notion that video spots -- skippable or not -- are responsible for declining CPC rates seems a bit of a stretch.

One thing is clear, though. Mastering mobile revenue is a challenge Google is still addressing, regardless of Pichette's explanation, and investors would be wise to keep tabs on its progress.

How much is too much?
There's a growing sense among some industry pundits that Google is taking its innovation efforts to the extreme. And as this article discusses, CEO Larry Page's stubbornness to truly shelve Google's unsuccessful Glass initiative is a glaring example.

What's the big deal? The aforementioned rise in expenses, combined with off-the-charts innovation development efforts, has left a bad taste for some.

Another example will likely be driverless cars. Sure, self-driving cars is a fun, Jetsons-like futuristic project, but it's been going on for years -- and who knows how much Google has already poured into the initiative -- and it will take years more to finish. The regulatory hurdles alone could take longer than the development cycle.

Does Google have the wherewithal to cut the cord on projects with little or no business case -- as intriguing as they may be? Time will tell.

None of these issues are reasons to rule out Google as a long-term investment. However, they are worth keeping an eye on.