Q: After moving to HTTPS, why did some pages lose rankings while others were fine?

Q: After moving to HTTPS, why did some pages lose rankings while others were fine?

Good eye spotting that it was uneven — that's a clue.

Short answer: the dropped pages probably have mixed-content or internal links still pointing at http.

Long answer: a clean HTTPS move barely dents rankings. When only some pages slip, it's usually that those pages still load assets over http (mixed content), or your internal links and canonicals weren't updated to https. Google then sees a muddle of http and https versions and isn't sure which to rank. Hardcoded http image URLs and old absolute internal links are the usual culprits.

Next step: crawl the site and filter for any http:// resource or link. Update internal links and canonicals to https, fix asset URLs, and make sure every http URL 301s to its https twin. The uneven pages will recover as they're recrawled.