Poor service

Hi a consultant of Mtn contact me that was sometime in last year for data contact for 40GB for only R299 pm and I accepted the contract the simcard wS was delivered and everything this 40GB was the sim but as the months went by i discovered i only get 20GB and not 40GB we called in we spoke the manager aswell Kim Basson but nothing was resolved they send an sms saying 40GB was sent to your number buy if i check then only 20GB is loaded and not once we skipped a payment and I'm tired with this story that cant be resolved