Woolworths and Nimble group

I had an outstanding balance. Woolworths handed over my account to Nimble group for debt collection. I made a total payment towards the account, yet the balance is still the same. I have sent numerous emails and proof of payments and requested a paid up letter. I am not getting any help from both Woolworths and Nimble group. This is frustrating because it is affecting my credit score. I now don’t know what to do or who to contact.