Multiple Incorrect Debit Order Reversals

We have an account with FNB that we mainly use for a specific function and therefore have only one debit order going off every month. There is usually enough funds to serve the debit order but sometimes I have to pay in an amount to cover it. I always make sure to pay the amount in advance but for some reason FNB has reversed this debit order twice while there was enough funds in the account. When I log a complaint, FNB totally ignores it and doesn't even get back to us. This is driving me crazy as I have to pay reversal fees, pay the account manually and probably get a bad credit record just because a bank, FNB, cannot do their calculations correctly when the debit order goes off and decides to reverse it. Is it still possible today for a bank to not be able to calculate balances correctly and then make you pay for it? Obviously, if you bank with FNB...