If the field groups are created using ACF then make sure that you’re following the steps outlined here https://www.advancedcustomfields.com/resources/upgrading-v4-v5/
If the fields are created in PHP then the code used to create them will need to be altered what needs to be done has changed. Some of these changes are listed here https://www.advancedcustomfields.com/resources/whats-new-version-5/
