SAN FRANCISCO — Google’s fourth-quarter earnings rose 17 percent even though a long-running slump in its online ad prices deepened.

The performance announced Thursday indicates that Google is still struggling to close the gap between the rates for ads shown on mobile devices and those on personal computers.

Advertisers haven’t been willing to pay as much to reach prospective customers on the smaller screens of smartphones and tablets, but Google Inc. has been tweaking its digital marketing system so mobile and PC ad campaigns are bundled together, hoping advertisers eventually will recognize the advantages of reaching people on the go.