Field symbol has not yet been assigned.

Hi,

I have a scenario where I am sending IDOC from R/3->XI->SCM(Proxy). XI received the IDOC and converted that to XML and sent that to SCM. In SCM to process that XML message it need to trigger/call proxy runtime, but it's ended with error "Field symbol has not yet been assigned" in the queue and message status is set to "scheduled" and no further processing is done. In the initial monitoring screen, I am able to see Technical Inbound channel as "IENGINE" and Outbound Technical channel is empty. I am getting the above mentioned error message only when Outbound Technical channel is empty and in the cases where Outbound technical channel is filled with value "PROXY", it's properly processing the message and XML message successfully got posted. I am getting the error only when Technical Outbound channel is empty. Can anybody explain why sometimes the Technical Outbound channel is empty and if it is empty what needs to be done to clear this and reprocess the message.