On-Page factors:
* Keyword in title tag
* Keyword in description tag
* Keyword in H1 tag
* Using keywords in the page's copy
* Length of the content
* Duplicate content
* Canonical tag
* Image Optimization
* Content Updates
* Outbound links
* Internal links
* Keyword in URL
Site factors:
* Sitemap
* Domain trust
* Server location
* Mobile optimized site
* Google Webmasters Tools integration
Off Page factors:
* The number of linking domains
* The number of linking pages
* PageRank of linking page
* Link relevancy
* Authority of linking domain
* Links from homepage
* Number of Dofollow vs. Nofollow links
* Diversity of link types
* Contextual links
* Link anchor
Domain factors:
* Domain registration length
* Domain history
* Country TLD extension
You are may be implementing Black Hat SEO techniques which is making your website rank to drop. Avoid keyword stuffing, duplicate content, cloaking, hidden text or urls etc.