Customers now paying for FNB transfers no longer clearing immediately

Good day. I don't understand something, do transfers betweeeln fnb accounts no longer clear immediately ???On the 25th I transfered money to my premier account for my debit order. The debit order returned back unpaid. When I saw additional debit I added money this still been the 25th. Still returned back unpaid. 26th I transfered money for another debit order. Same story. How is it possible that fnb keeps charging me these ridiculous unpaid fees when there is funds in my account for debits. Every month it's the same story. Why are transfers no longer reflecting immediately. It's taking 24hours for them to reflect yet I'm been charged these ridiculous fees every month