A little over a year ago Google implemented its "mobile friendly update" (a.k.a Mobilegeddon). This update made the mobile compatibility of a website an actual ranking signal in Google's algorithm. Most of the discussion leading up to the update was the importance of websites being "responsive." This caused a lot of businesses to scramble to make their sites responsive, or it created a false sense of security for those whose site was already responsive. There is a little twist to the story...
Being Mobile Friendly is Pass or Fail
Google does not rate "mobile friendly" in degrees. It is pass or fail. This means that if you have an image hanging off the side of the screen (event slightly), that entire page is considered "not mobile friendly." So even if you have a responsive website, it is quite possible Google will consider some or all of your pages to not be mobile friendly. You can test your website's mobile compatibility with Google's Mobile Testing Tool.
Here are the most common issues we have found that have prevented a responsive website to be considered mobile friendly:
1) Make sure you are not blocking template resources.
This one is a little technical for those who do not build or manage websites.There is a file on every site that sets the rules for non-human visitors. Among these non-human visitors are Google's indexing programs (a.k.a. spiders) that scan your site and index its information. Many template frameworks may, by default, block access to template resources that make up how the site looks to the human eye. These resources may include important formatting code (CSS) that makes your website mobile friendly according to the new guidelines.
SOLUTION: Edit your robot.txt file to allow non-human visitors access to the CSS files and other template resources.
2) Touch elements (links, buttons, etc..) are too close together.
If access to template resources is not the problem, then you probably have elements too close together for mobile users.
SOLUTION: Make sure all touch elements have at least 48px spacing between them. This is most commonly a problem with sidebar menus, or lists of links that are not altered for mobile screens.
3) Content wider than the screen.
This is more common in websites that were retrofitted to be responsive, instead of being completely rebuilt. It is most likely an image/artwork that is overriding the global formatting to make images responsive.
SOLUTION: Brute force.
If your website is not responsive, then it is absolutely time for an upgrade. Competition is getting stronger in all markets for search engine placement, and the factors affecting your SEO are becoming more complicated and difficult to control. This is one factor that you can, and should control.