MTN is PATHETIC

I have a contract with MTN where I am supposed to be getting 30G per month (20G anytime data and 10G midnight data) every month. But instead I only get 10 (5G anytime data and 5G midnight data). I went to the Maponya mall MTN branch for 3 consecutive months to enquire about this and everytime I go there, they tell me that they will try to sort this out but they never sorted this out as they had promised. It's been 7 months now and the problem is still persisting, they are not doing anything. And what saddens me is the fact that they debit money from my account every month but I'm not getting the service I deserve. I am really disappointed.