Dan

Problem:
Custom fields were not available for translation.

Solution:
- Set the required custom field's translation preference to translate or copy as required from wp-admin -> WPML -> Settings -> Custom Field Translation

- If you don't find the field there click on "Show System Fields" just below the "Custom Field Translation" heading.

- Edit and update the page with the issue in primary language.

Relevant Documentation:
https://wpml.org/documentation/getting-started-guide/translating-custom-fields/