Netinstall image creation may not occur if the system version of the image is significantly newer than the system version running on the server. For instance, if you try to create a Mac OS X 10.4 image on a server running Mac OS X 10.3, you will see the message "A bill of materials (BOM) file required to create this file image is missing from the source" and the image will not be created.
Make sure that the server is running the same reference release (for example, 10.4.x) as the Netinstall image you are creating.