fnb make me sick

I noticed that a debit order of 318.99 didn't go through on my account yet I had money in account for that debit order , now fnb has charged me an additional 100 - this is nonsense man - why are fnb such *****s in that they take away money from people without even asking - I want this R 100 get - I have enough money in my account to have paid for this amount- this is totally out of line -I always have issues with FNB honestly