If well-ranked sites suddenly disappear from the search machine index, it can be for a number of different reasons: In one case, the crawler could no longer check the pages in question, or in another case it could have identified duplicate content and removed one of the pages from the search results. Another reason, often unknown to the external developer, is so-called URL hijacking – a process in...

Next I would look at the Exit rate (The percentage of people that left on a given page). The Exit rate is useful for identifying common pages that people are leaving your site on. This often points to problems on the page or shows you that people are not finding what they want (for example if a search result page is a common exit point). I order this report by Exit rate, filtering out outliers first.
"Mobile is critical — Google has finally noted that more than half of searches are conducted on mobile, and this will only go up as computing becomes even more ubiquitous," Noah Jessop, head of data for Liquid PCH, told Marketing Dive earlier this year. "The shift to mobile is only going to increase — and unprepared marketers will be left trying to catch up."