Debot order dates NOT changed after request

On 13 Feb 2020 I requested that ALL my debit order dates on my FNB account be updated to a new date as I had a change in salary date. I received communication from *** asking for a letter to confirm the change. This was submitted to them. It was then asked from which date this should be done and I specifically indicated on email that is must be from end of March. So this morning (27 March) I had to see on my bank account that FNB tried to debit my account.... after requesting the above changes. This is absolute BAD service.... I have been trying for a week now to speak to someone at FNB regarding other stuff as well with no luck. No response on follow up emails, no answering of calls.... What am I to do now?