Incorrect credit rating

I took a loan from OMF and at that time I was getting paid on the 15th of every month. I then changed the employer and now get paid on the 25th of every month. Necessary arrangements was made with old mutual only to find that the debit was still going off on the 15th despite my request to have the debit date amended. I have never been in arrears with my payment however I was receiving calls from old mutual saying that I am behind, this went on for 7 months regardless of going to the walk-in centers and requesting the debit date to be changed. The worst nightmare now is that when I requested my credit report from transunion I was livid to learn that old mutual have now listed me saying that I am behind with my payments. I have since never received any call from them to correct this as it have a negative impact on my credit rating. this is so unfair particularly if you have been loyal with the institution. I is very disappointing that such a big institution can be so ******** and render such a poor service.