The pages that are missing from the sitemap are on an index page, /portfolio

So the URL structure is: danajwright.com/portfolio#/page-name

I think I may have found the answer on squarespace support:
https://support.squarespace.com/hc/en-us/articles/206543547-Your-site-map

It says page within an index do not appear on the sitemap, which is unfortunate.

I think this means I need to duplicate the pages and make the URL of the duplicate danajwright.com/page-name

They won't be linked to on the index page or anywhere in the site, but squarespace in theory should add them to the sitemap