If u pm me your website adress i can tell you what the problem is within a few minutes. I am not active in the blogging niche and have no interest in it. I focus on quinstreet and mainly leads, comissional income and passive adsense informational websites. There's too many factors to discuss here; there are literally thousands pages in books and articles that adress this very problem.
Some main geustimates:
- Spam links / PBN found = site manually punished, possibly not recoverable
- Poorly spun content, paid content
- Found paid traffic
- Rankings loss
- Backlinks devalued.
- Manual review deemed site lower quality
- New theme, poor or wrong SEO practices
- ...... and so forth...
