DELIVERATE INTEREST INCURED FOR ME BY FNB

FNB Has been debiting a lesser loan installment amount on my salary that is paid in their account .This has incurred me interest and service fees and arrears ,iv been trying to resolve this with their secure chat however i am not getting a solution as the error is form their side, all they keep saying is that they will debit my account, again .i am really unhappy as i have a loan agreement with them that stipulates ,repayment amount and date. they have gone against the agreement and now i am suffering the consequences.