Symptoms
Workflows are not firing when a new item is added, even though the process is set to start On Item Added.
Diagnoses
We looked at the event receivers (using the SmartObject Service Tester > SharePoint 2013 > [Site] > Management > Event SmartObject, executing the Get Event Receivers method) on one of the libraries and saw that the remote event URL for the ItemAdded receiver was set to http://[server]/runtime/runtime/SP15EventService/RemoteEventService.svc, instead of http://[server]/SP15EventService/RemoteEventService.svc.
Resolution
We checked the Environment Library and saw that the default Web Service URL environment field was a field called "Smartforms Runtime" of type "Web Service URL" that contained the Smartforms runtime URL. We set it to the correct Web Service URL field (http://[server]), removed the event receivers from the process, deployed the process, re-added them and redeployed.