MTN REFUSES TO BELIEVE DATA CHANGE ON CONTRACT

I took out a new contract with MTN in March. I needed a specific amount of data, therefore chose Made for Me M which came with 3GB. On top of that, I added a 2GB Data Booster. For March, April and May, I received the full 5GB (3+2). Suddently, in June, I received only 1MB with the contract, and then the 2GB booster. Its WRONG. The SMS still say "Yello, you have been credited with 3GB data" but we only get 1GB. Next SMS says "Yello, you have been credited with a 2GB Data Booster" which is correct and I do receive it. MTN has admitted to me that they made changes to their contracts in June. Surely this should not have affected us. MTN Clearwater is unwilling to assist us and cannot see the problem. I have called MTN 4 times and the person who promised to assist me, Luyanda Cebekhulu ignores my emails completely. Why does nobody at MTN believe me. MTN your service is shocking, I am at a loss.