It should be noted that in Purchase Requests, you do not see the nominal codes themselves. You see the Purchase Ledger Analysis codes and it can be common for this record to not be created when a new nominal code is added. Please check that this/these codes exist there.

It is likely that your Purchase Requests are listing the analysis codes from a view or stored procedure. If so, there will be code in those to determine which codes are returned. Log a case with Support who can review these area and advise on the reasons for exclusion to for future reference and system management.