QOSAddSocketToFlow returns error - ELEMENT NOT FOUND

Hi

I am working with QWAVE api to set DSCP value in packets.

I am trying to do multiple calls with DSCP bit set as 46 between server and client. For each call I have to add socket to the QOS flow which is done by QOSAddSocketToFlow.

Problem arises when DSCP bit for some of the calls were not getting set correctly.

Further I found that for those calls QOSAddSocketToFlow api is returning an error code
Element Not Found

This error is no where specified as possible return code in msdn site https://msdn.microsoft.com/en-us/library/windows/desktop/aa374027(v=vs.85).aspx

I am using QOS_NON_ADAPTIVE_FLOW and traffic type as QOSTrafficTypeBestEffort

QoSResult = QOSAddSocketToFlow( QoSHandle, ConnectSocket, sockaddr, QOSTrafficTypeExcellentEffort, QOS_NON_ADAPTIVE_FLOW, &FlowId);

I want to understand in what scenario QOSAddSocketToFlow can throw this error code. Also, this is not happening for all calls, some random calls are getting this error.

Any help will be appreciated.

Thanks