Maintaining your position in Google SEO rankings is no small challenge. As from April 21st 2015 Google has changes it’s algorithms to reflect the growing change of information consumption on smartphones and tablets.
Therefore if your website does not meet Googles “mobile friendly” standards then you may well find yourself slipping down the rankings.
Quote from Google
More mobile-friendly websites in search results
Starting April 21, we will be expanding our use of mobile-friendliness as a ranking signal. This change will affect mobile searches in all languages worldwide and will have a significant impact in our search results. Consequently, users will find it easier to get relevant, high quality search results that are optimised for their devices.