I don't remember that I did any thing except I started to raise my posts words, I mean editing my top posts from 1000/word to 5000/word with good quality content. I don't know if that was the reason or something else, I would be glad for any advices.
Analyse your website according to recent Google update because its updated algorithms again in march and CTR going down means you are not going more friendly with search engines. I think it can also be due to many display ads .... I will suggest you to use Auto selection to display ads because if we will try to put ads forcefully then may be we are avoiding some rules or guidelines because you are thinking about more and more money by advertisement and search engines know it very well that what you are doing