Why does my installment loan not show up on one credit report?

I have had a car loan for a just about two years and when I got my yearly free report from TU it was on the report and it said it was "open, never late", now a few months later I get my free report from Equifax and this loan is no where to be found. I haven't paid it off or anything yet, it is obvious they are just unaware of it. My question I guess is how do I inform this company of this loan, which I am sure will help my score?