[Koha] Problem in importing Barcode Numbers

Dear list members, We are converting our MS Access data to KOHA. Data from different fields was successfully imported to KOHA in bulk but ITEM INFORMATION (BAR CODES) are not being imported by the bulk code instead null values are shown for bar codes in imported records. Can anyone suggest what is the problem and how it can be handled. Yours, MIRZA MUHAMMAD NASEER Pakistan.