If you get NICS delayed on every firearm purchase, the reason is most likely that you have something on your FBI Identity History Summary that must be researched in order to allow an approved transaction.
The reason why you get delayed on every submitted transaction is because the NICS Section is required to destroy all identifying information on allowed transactions within 24 hours of notification to the FFL, thus requiring research on every subsequent transaction.
To allow the NICS Section to retain identifying information for the approval in the Voluntary Appeal File (VAF), and preventing delays on future transactions, submit a VAF Application to the FBI.
VAF Application found here: https://www.fbi.gov/file-repository/vaf-form-25.pdf/view