Fluctuations are completely normal and you may have to wait a little longer for the rank to be stable again. I can also suggest that you increase the number of backlinks to this page to ensure that you attain the required results.
Please note that ranking on Google depends on so much more than just your on-page optimization. You could have the perfect article and still struggle to break the top 10 in SERPs. Here are some of the factors Google considers when ranking you:
1. Your website’s authority
2. Your content’s quality and how better it is than the other article currently ranking.
3. How competitive the keyword is.
4. How many backlinks you have and if they are better than what the other guys ranking for that keyword have.
5. Your website’s crawlability.
6. Frequency of new content and content updates.
7. So much more that there is a dedicated blog post on it by Brian Dean 🙂