Why is Google not returning all reviews?
Google has a limit on returning reviews beyond ~4,500 reviews, meaning that even if the business has 10,000 reviews, we can only provide the latest 4,500. Unfortunately this is a limit with Google and there is no workaround. We suspect Google has reviews stored in cold storage, and it's not often that a user would ask to see that many reviews.
We are constantly monitoring this and working to be able to return all reviews for all the review sites we work with.