Cannot think of any specific reason.
Have you configured the partner profiles? The easiest way to check would be to take the idoc #, go to WE19 and clone the same. Click on the standard inbound. You should get a pop up. At the bottom there should either be a green or red light which will indicate whether partner profile is set up or not.
another option is to go to BD87 and reprocess the idoc and see the error message.
Thanks,
Vikram.M