Fnb does this every month they debit my bank account,the transaction is successful,there is funds in the account at all times especially at month end …

Fnb does this every month they debit my bank account,the transaction is successful,there is funds in the account at all times especially at month end …. They then send a message a day after debit has bounced and then charge me R225 bank charges etc for unpaids. I want my refund immediately and I want a manager to call me to explain why does this happen monthly? No other company that debits my account ever has this issue it’s always FNB!