Back in 2012, the sky fell when Google released a penalty for over-optimization. It’s not often talked about anymore, but the updates are still in place. Over-optimizing your site/content can actually harm your rankings.
When doing on-page SEO, it can be enticing to add your keywords everywhere. Keep your content natural and write the way you speak. Over-optimization occurs when you include your keyword too often and use keyword-rich anchor text for all your internal and inbound links.
It seems counter-intuitive, but if you have trouble ranking, you may want to optimize less.