I have 6-7 pages that used to rank until Mid-October. I have a main article and several sub-sections covering multiple longtail keywords. For example, if my main keyword is "keyword research," then I've covered keyword research tools, keyword research for beginners, keyword research for youtube etc. in sub-sections. I think Google implemented recent spam update to get rid of such content. The competitors used the keyword only 2-3 times in entire article. So, I think using too many variations of the keyword is also a problem. You should update content to reduce keyword density and see if it works.