TN getContentPart problem

Hi,
I am adding a content part with a part name(say payload) and MIME type(application/EDI) to a stored(already processed, retrieving the data using restorePipelineFromFile) bizdoc using the addContentPart service.

Later when I use the getContentPart with the same part name(payload) its giving me content part as ‘null’. Am I doing something wrong here please help.

Thanks,
wmdev1212