Description 
print-03 (or other Service outputting bib records): when including z30 item record fields in the bib record (via expand_doc_bib_z30) and when the number of items attached to the bib record is great enough to make the output record exceed 99,999 characters (as in the case of certain serials), fields are omitted from the output record. 

Resolution 
The Logical Record Length of a MARC record is 5 characters, which means that the maximum size of a MARC record is 99,999 characters. 
It's likely you will also be exporting loans (z36), cash (z31), and other circ tables. Assuming so, -- rather than exporting the items for these serials as part of the bib record -- you could export the z30 item records in exactly the same fashion -- and then use that for the cases where the item information is incomplete because the record is larger than 99,999. 
The Service for exporting a particular Aleph Oracle table is file-03.