Woolworths milking pensioners on closed accounts

My mother closed her account in January 2020. She went to the store a couple of times and she was assured that its closed. But this account is still accumulating interest its showing fhat is in arrears. How can be she be in arrears when she closed the account. Woolworths come one, any amount that might seem small to you causes stress to a pensioner takkng into account the number of trips they must make to sort out one thing.