MTN cheating me with data package!!

What a joke... For the past few months I've only received a quarter of the data I pay for on my rooter! I'm paying for 40gig Day and 40gig night data per month BUT am only getting 10gig day and 10 gig night data... How is this happen MTN?!? I have been to the local MTN branch about 4 times and according to them they are not getting feedback from head office. Apparantly I'm not the only customer being cheated like this. Utterly unacceptable!!! I either want the money back I had to spend because I didn't get what I pay for or all my data credited to my account!!!