Send them proof of your cancelled check along with the back showing it was cashed.
Many of the clerks will not take the time to investigate your account and straighten things out and next thing you know, they'll report you as delinquent to the credit agencies.
A complete pain in the rear. Chances are they applied your payment against the wrong account and can't find it nor will they do much leg work to try to find it.
"The farther we go, the more the ultimate explanation recedes from us, and all we have left is faith."