Create KB article button missing where Service Desk issues not crated via Customer Portal

Hi,

We're using Confluence 5.4.3 with Jira 6.1.6 and Service Desk 1.2.1.

In the edit issue view in Jira, the "Create KB article" button shows up inconsistently - some issues have it, some don't.

They're all in the same Service desk enabled project, of the same issue types, and there seems no obvious pattern about when they were created/edited/resolved.

Please see attached screenshots of two virtually identical issues - one that displays, the other that doesn't.

Thanks,