I have one site with all the articles are copied from other blogs and websites. I have written source for each article. At the first time, it's ranked quite well, but now all the articles are down to the end of SERP. I don't know why. I doubt it causes by the duplicate content. Why can explain? Thanks so much!
Google will slap you hard if they find pure duplicate content on your site. It maybe because the other sites have improved on their SEO. My tip : Add a bit of your own content with it