To rank in the top 1 again you can do some things. Start to keep an eye on your competitors and how they make a difference and what methods they follow. Create as many high-quality backlinks as you can. And keep following the strategy you made to rank on SERPs.

Do you see any difference in visitors behaviour? Like lower avg time on page, higher bounce rate, ...

If for some reason large number of visitors bounce (pogo sticking in this case), it's good enough reason for Google to re-evaluate your page and move it (down).

Ensure an internal page link from the key phrase as anchor text. Also, win 1 backlink for the key phrase with brand name included. Moreover, you can optimize the alt attribute of the images on the page with the key phrase or parts of it included.

Hi there. I have a coupon site with 100k+ traffic monthly. One of the top pages ranked down from No1 to No9 last month, which caused the overall traffic drop of my site. After I analyzed the No1 ranked page with the same main keyword, I optimized the content by adding the latest coupon information and unique content and inserting the main keyword properly to keep the keyword density. However, the ranking only increased to No7. Is there anything else I can do to help improve the ranking? Thank you in advance for your suggestions.