Credit missing

I returned a jacket at yde centurion in November and subsequently received a message saying I owe them and need to make a payment.when I went to the store to make a payment ,I told the cashier I did not owe any thing on my account but I got a msg to make a payment.it turned out that my credit was never made to my account.They are still trying to locate the credit.This is very poor service.i have to call and follow up.how can the credit disappear.