When attempting to add a related list (Configure > Related Lists) to a form, the expected entry does not appear in the "Available" list of the slushbucket.
All supported releases.
A list will not appear as an available option for a related list if the same list is already present on the form as an embedded list.
For example, in an OOB instance the Impacted CIs related list is included on the incident form. If the same list is added to the form layout as an embedded list, the related list will no longer appear and will not be an available option if the user attempts to re-add it.
Remove the embedded version of the list from the form.