Google's April 21st 2015 change to their search algorithm seems now to be programmed to favor search results that are prepared for mobile display when searched on a mobile device. If one's website does not have a mobile version for a particular search result, one could drop below those that do. Mobile-friendliness is now counted as a ranking signal when deciding who gets the top spot.
This means that you could rank on page one for a search term when someone searches on their laptop or desktop computer, but fall off the grid when the same search is performed on a mobile phone or tablet.
To check if your website is mobile-friendly and/or see whether or not your site could use some adjustions, visit: www.google.com/webmasters/tools/mobile-friendly/