MTN contract that ended many years ago shows on their systems as being arrears.

I am disappointed with MTN, I went to apply for a contract at an MTN store today and I was told I could not apply because a contract I had with MTN that ended many years ago is preventing me from applying for new contract. Back then I notified MTN very early that I would not renew the contract after 24 months because of loss of employment but after so many years it shows on their systems as being in arrears when they enter my ID number. How does a big company like MTN have such poor systems?