FNB is becoming a bank of frausters

FNB has the tendency of processing debit orders late and also reject debit orders even if there are funds available. On numerous occasions debit orders have been rejected even if there are funds on my account or processed late. Initially I was told that the system is rejecting debit order. Now I am being told that some purchases were updated in between my debit orders which was banked by the merchant few days after the purchase was processed, hence my debit orders were declined. How can purchases that are already reserved affects debit orders? This other instant ( I think it in May 15)the debit order was first paid and few days later than rejected. This is day light ******* and needs FNB needs to be investigated on how they process transactions.