I have a suggestion if anyone wants to follow it. I am in a niche where, to my embarassment, mine is the only site to drop off of page 1 for my major term -- I also fell anywhere from 5 - 20 places for dozens, maybe hundreds of other terms.
My competitors all seem to be doing fine. So why have I fallen and they have not? My site is every bit as good as theirs in terms of content, when a human being reads it. In fact, far better than several of them.
I am going to do some more research on this later today. But I will be the first to say I am not the best at this kind of research, not being a web developer. I did not build my site. I know quite a bit about SEO, but I'm geared more to the public relations side of things. I am a writer of content, more than anything (which makes my current situation kind of ironic, no?).
Anyway, if anyone else wants to use my site and niche as a test example, just let me know and I will give you the niche and the keywords etc. As I said, it provides a test case where just one site out of the top dozen or so has been hit. Since most of the sites have similar content, there may be something about my site that I am overlooking, but which a more expert SEO would spot quickly.
Let me know if interested.