Maintain internal linking to your old blogs!
What happens, on many sites, is that pages initially get crawled then indexed from the blog’s homepage.
However, they fall off the homepage as new content is published. Many blogs then rely on a JavaScript button to load more pages. Google usually does not interact with this, so you lose key internal links pointing to articles.
They remain indexed since they are in your XML sitemap, but this leads to a large decrease in keyword rankings for your past, yet possibly still valuable, content. Without links, Google assumes it’s not important.
The best solution is to use pagination-based links in the HTML on the blog homepage. If this isn’t possible, you can perhaps boost internal linking directly between important articles.
Finally, you may want to consider a more complex web dev fix like creating an IntersectionObserver, which also helps search engines find infinite scroll-loaded content.