Find out who outranked your brand in search. And how many blogs have dropped in rankings.
See if your search competitors have increased their content frequency, topical authority or tight interlinking (based on themes or hub & spoke model)
Check GSC reports (impressions, clicks, avg position) for first-party data. Get the metrics for your entire blog and also for individual blog posts. See if there are any seasonal drops or a common pattern.
Just like @Aniruddh Jain said, for some topics Google prefers fresh content - Make smaller changes like including a couple more pointers to blogs every month, adding more images, alt texts, and interlinking possibilities. Do a complete content revamp if the content type, format & angle doesn't satisfy user's intent.
If you are updating or new adding blog posts, try including schema/structured data to it based on the content format. It can improve your CTR and featured snippet positions.