Debit orders going through on incorrect dates

I have an agreement with MFC that debit orders must go through on the 20th of every month. This month the 20th was on a sunday so they debited on the 19th of which there was nothing on my account and it bounced as i get paid on the 20th irrespective of what day it is. I qas so angry i phoned them and they said they will fix it i must make a direct payment of which i did on the 22nd. I phoned to ensure they did receive the payment and they said yes. On the 24th again they did the same thing after i even made a payment , they debited and it bounced. Now i have to RD (return debits) on my bank profile as if im not a good payer. I reported this to my Bank and they said i must get a letter of apology from MFC so that they can try fix my profile and remove the RDs. I've been calling and calling, sending emails to MFC and no one is assisting me with the letter. When i ask consultants to call me back as im currently on leave and dont have airtime anymore, they promise but dont call me back. Im so frustrated as this whole thing has affected my bank profile so bad and has caused me a lot as i have to use my own money, time to get help and no one is helping me just to get a letter.