Multi-value Custom fields not showing up for export
Multi-value (also called multi-record) custom fields can't be exported by the normal export. See https://docs.civicrm.org/user/en/latest/organising-your-data/creating-custom-fields/#multiple-record-fieldsets.

However you can get round this using a report then exporting as CSV. Two caveats. Not all reports will include the multiple values. Constituent detail doesn't allow CSV export, but I have found constituent summary to work for all I need.