You are unable to lookup subscription items under the Orders accordion in the Acquisitions module when searching by OCLC number when the OCN has been merged into another record.
If you find a title that has a "(Merged Record)" message following the OCLC number in an order, use the Resource history link you can click on in the upper left corner of the General tab inside the Order. This link leads to a page that will give you the "Item Identifier" or OCN that you can search to bring up the title.
There is more on transferring data for merged serial titles.