My boss and instructor once told me, dont believe anything the insurance compny tells you
When we looked at the remittance, I would call the insurance companies that had the claim crossed over or sent to the secondary. The usual response 100% of the time was , we're sorry, the claim is not on file. So,we sent all secondaries ourselves. If there was more than 1 claim, they went certified mail/return receipt. Even doing that caused, we're sorry, the claim is not on file. That didnt matter, we had the certified mail/receipt to send to the regulatory authorities which slowed the NOF excuse.