Unable to change defect status by REST API

I saw some guys are able to update state by oslc API, but I am still not able to do that.

I get http 200, but the status of defect is not changed. I am using JAVA. Can anyone help me with this? Here is my steps:

1. HttpGet

url: https://server/ccm2/authenticated/identity

2. HttpPost

url: https://server/ccm2/authenticated/j_security_check

3.HttpGet

url:https://server/ccm2/resource/itemName/com.ibm.team.workitem.WorkItem/19917

"Accept", "application/rdf+xml"
"OSLC-Core-Version", "2.0"

then get the "ETag" in response header.

4.HttpPut

url:https://server/ccm2/resource/itemName/com.ibm.team.workitem.WorkItem/19917?_action=com.ibm.team.workitem.taskWorkflow.action.startWorking

"Content-Type", "application/rdf+xml"
"If-Match", ETag
"Accept", "application/rdf+xml"
"OSLC-Core-Version", "2.0"

with the response xml in step 3.

 I also tried "https://server/ccm2/resource/itemName/com.ibm.team.workitem.WorkItem/19917?_action=com.ibm.team.workitem.defectWorkflow.action.startWorking", "https://server/ccm2/resource/itemName/com.ibm.team.workitem.WorkItem/19880?_action=com.ibm.team.workitem.taskWorkflow.action.a1", "https://server/ccm2/resource/itemName/com.ibm.team.workitem.WorkItem/19880?_action=com.ibm.team.workitem.bugzillaWorkflow.action.startWorking" and "https://server/ccm2/resource/itemName/com.ibm.team.workitem.WorkItem/19880?_action=bugzillaWorkflow.action.close"

All return 200, but status not updated.