Item paid no fund ****

I'm trying to understand the unpaid item fund how does it occur when there was money in my account and if the items where unpaid why the fee doesn't include the money for the item that u claim were unpaid, let's say a R100 is unpaid why not charge the R100 plus your R45 fine, and no money came in my account after I made a purchase which I believe if there wasn't enough money my card would decline. Fnb is really ****ming us