If you use Google Webmaster Tools, which you should do, you may receive an email from Google with details similar to those below.
Fix mobile usability issues found on http://example.com/
To: webmaster of http://example.com/
Google systems have tested 69 pages from your site and found that 100% of them have critical mobile usability errors. The errors on these 69 pages severely affect how mobile users are able to experience your website. These pages will not be seen as mobile-friendly by Google Search, and will therefore be displayed and ranked appropriately for smartphone users.
This warning is part of the huge effort Google is taking in assisting websites to become responsive (look good on iPhone, Android and other mobile platforms). In case you’ve missed the biggest SEO news for along time, Google has made it known that they’ll favor mobile-friendly websites in mobile searches from 21 April 2015.
First off, don’t panic if you get an email such as this. It is important to take action however and to ensure that the issues are resolved so that your website doesn’t suffer in the search engine rankings due to mobile-friendliness problems. In the email that you’ll get from Google Webmaster Tools, they provide various links to identify and troubleshoot problems.
If you use WordPress (or another CMS) it can sometimes be difficult to identify why Google is having issues (or more precisely, how to fix issues identified). In some instances a plugin may be interfering with how Google sees your website or Google may just be relying on problems you’ve already fixed on your website. In a worst case scenario, your theme may not be mobile-friendly which may mean a theme change.
All in all, it’s very important for SEO that Google identifies your website as mobile-friendly so it’s important to spend time addressing problems found by Google. If you need assistance in fixing any problems feel free to contact me.