Symptoms

a SharePoint 2013 process that is set to start when an item is checked in. The process does not kick of when the document is checked in. Starting the workflow manually does work.
 

Diagnoses

checked the /sp15eventservice/remoteeventservice.svc url and it is accessible.

Verified that the integration is visible in the DB's
 

Resolution

If you are using SharePoint online then Event Receivers will only work with a purchased certificate, and your K2 environment URL/end point needs to be accessible by SharePoint Online.
Check out http://help.k2.com/KB001443 and view the Certificate and Port Requirements at the bottom of the article.
I think the reason why the manual trigger works, is because the page that you see to start the workflow is a SmartForms page running on your K2 server and therefore it knows how to start a workflow on itself.