There is more than one credit agency so perhaps it is showing up on one you have not checked. You can review all of your credit reports (free of charge from the link below) to check whether the company is reporting your account to any of the agencies.
https://www.annualcreditreport.com/index.action
If your account does not appear on any of your reports, there is really little you can do beyond appealing to the company to report your account and history of payment.