The attendant algorithm change took effect on April 21st. It was a very big shift that affected a lot of sites and has come to be known as "Mobilegeddon".
If you're not sure if your site has been affected then you can check your website for issues here. This test is not totally accurate, apparently. You can get a pass but still have some issues that cause ranking drops. But it's still worth using in my opinion.
All of my sites are with Weebly, Blogger and Yola, which have all been ahead of the curve with this. The templates I've been using are all mobile friendly so I was hardly affected at all. There was one site that had issues, though. I found out via the dreaded Google Webmaster Tools message. Lucky for me it was totally due to the tag cloud that I had on the site. Pretty much every page was cluttered with tags.
So what I did was just limited the tags to ten, and the mobile issues disappeared. Was interesting to see the graph in Webmaster Tools. After I made the adjustment the line just sloped down from hundred percent of pages with mobile issues to zero over several days. The search engine spider's visits were collating that data, but not all in one hit.
I was lucky. But a lot of people who've had their sites custom-made were hit hard. Don't envy them, that's for sure.
As far as I'm concerned this has been yet more confirmation that going the DIY website route has advantages. Even if you don't master the art of website building from scratch, you certainly can't help but learn quite a bit. You can then end up solving some issues yourself. If you had no knowledge at all, and no sense of ownership over the process, you can end up forking out a lot of money to have such things fixed by others.