Custom theme not working after update to Magento 2.3.2

Hi am upgrading a store from Magento 2.1.9 to 2.3.2, and I’m having issues with my current theme, which I created for the existing site. On my dev server, the site works as expected, however on the live server, the theme’s default.xml and default_head_blocks.xml files are ignored. The only difference between the 3 servers (Live, Dev, New) is that the live and dev servers have the same document root path as each other, but the document root path is different on the new server, but I cannot see how this would cause an issue.