MTN debited my account but MY 50GIG was not loaded - Account number A***

I took a contract with MTN in January 2017 and all has been well till August, I am suppose to be debited R269.00 every month and the promotion I took was the one to load 50Gig for the next 2 years. Now the debit order went through as usual and only 2Gig was loaded in August as per the call centre agent. I requested for manager where I was advised manager cannot assist me as well. This is very unprofessional from MTN as I was promised a text with reference number but I am still waiting.How do I do follow up and what do I need to do to get the service I am paying for???? PLEASE HELP !!!!