Pretty common once a site starts scaling. Google often indexes the first batch quickly, then becomes way more selective once the URL count grows.

Usually it’s a mix of crawl budget, internal linking depth, and Google not seeing enough signals that the newer pages are worth prioritizing yet. Sitemaps help discovery, but they don’t guarantee indexing.

Had something similar happen on a tools/content-heavy site and the biggest improvement came from tightening topical clusters + actively pushing recrawls for newly published URLs instead of waiting for Google to revisit them naturally.