Knowledge articles attached under Related articles related list on a catalog item are displayed on the service portal when opened the same catalog item.

• The knowledge base, which the KB articles belong to is not attached on the service portal record.

• Check the Knowledge base name, assigned on the KB articles.
• Add that knowledge base to the service portal under the Knowledge base related list.
  • https://<instance-name>.service-now.com/nav_to.do?uri=sp_portal.do?sys_id=<affected_service_portal _sysID>