If you are using Activiti 6 you probably need a tenantId to deploy any resource. I'm not sure that it could solve your problem but I suggest to try the following potential solution.
If you take a look at the unit test included in the APS SDK, based on Activiti 5 Enterprise, you will find a complete example related to a generic Four Eyes workflow:
Below the snippet for deploying a process definition:
repositoryService.createDeployment().addClasspathResource("apps/fourEyes/bpmn-models/4 Eyes Principle-9011.bpmn20.xml")
Map<String, Object> processVars = getProcessInitVariables(String.valueOf(user.getId()));
Map<String, Object> taskVars = new HashMap<String, Object>();
ProcessInstance processInstance = runtimeService.startProcessInstanceByKeyAndTenantId("fourEyesPrinciple",
As you can see you may need also the tenantId for creating a new instance of your workflow.
Hope this helps.