It's hard to imagine Google (GOOG) having a search engine optimization problem — its name is the verb for "Web search" for hundreds of millions of people around the world.

But when it comes to Web research that borrowers use to compare different mortgage options, there are brands that are better known — and that's a problem for Google as it attempts to target that space, according to Greenlight, a digital marketing and research agency with offices in London and New York.

Greenlight says that as of mid-December, Google's mortgage comparison service does not appear at the top of sponsored search results for all mortgage-related terms. Google recently launched a U.K. mortgage comparison service allowing consumers to compare 5,000 different mortgage deals in a single search. The comparisons list products that are available from lenders and via brokers. Google also offers car insurance and credit card comparison services.

"Google's got a good service and it works very well. But regardless of that, it isn't getting the click-throughs," says Andreas Pouros, chief operating officer at Greenlight.

Greenlight says research shows that Google's entry into credit cards and car insurance had less than a 1% impact on what people clicked on when they searched Google for those products, and initial evidence suggests that trend is also true for mortgages. Even though people are searching for mortgages via Google for comparison sites, they aren't clicking on Google's own comparison engine.

Pouros says it's an issue of branding — people are obviously familiar with Google, but don't see it as a financial services brand, at least for rate and product comparisons. "It doesn't benefit from any specialized branding," he says.

Greenlight's research is primarily focused on the U.K., where Pouros says there's a mix of well-established mortgage comparison sites that have good brand awareness among the public. Companies such as Compare the Market, Go Compare, Money Supermarket and Confused all flood the British airwaves with ads that rely on humor. Compare the Market's ads, for example, feature Aleksandr Orlov the meerkat, who complains that people are typing the URL wrong, and are arriving at Go Compare's ads feature an opera singer. "It's hard to watch television for a half hour here and not see an ad for one of these companies," Pouros says.

Google's had similar problems with its U.S. mortgage search site. First, the site limited its operations to certain markets, then it shut down entirely earlier this year. The shutdown in the U.S. was limited to mortgages. Google continued to operate comparison sites for credit cards, certificates of deposits and savings accounts. Brand recognition was not reportedly part of the U.S. struggles. But, similar to the U.K., there are a number of recognizable mortgage search brands in the U.S., such as, Zillow, Bankrate and My Bank Tracker.

Google did not provide an executive for an interview, but issued a response to Bank Technology News that was more focused on the mortgage comparison service's competitive positioning than on branding. It contends the site's stable of 5,000 mortgage products compares favorably to its competitors, and that its site includes a "code of conduct" that is beyond what is required by the Financial Services Authority, the U.K.'s banking regulator. The broker's commission doesn't surpass 1% of the mortgage's value; and the lender and broker only pay when a customer clicks through to their website or calls.

"Over the last few months we have launched new comparison services for credit cards, savings accounts and car insurance in the U.K. We are now adding mortgages to our comparison family, making it easier for customers to find the best deal for their individual circumstances," said John Paleomylites, product director for Google, in a statement.