MM_EKKO has cusomizing involved. Per document type and item category you have to define a residence time 1 + 2 .
In any case, whatever archiving object you execute, always check spool file and job logs for more information. I am certain SAP is not speachless and logs the reason why a document is not archived in the spool file.
Of course you have to select the option to get a log in the selection screen of your archiving run. But this should be obvious when you start testing a new object.