Less Data than what is contracted for

For 3 months MTN has given me less data than what the contract has stipulated. Month 1, I tried calling them, sent them emails. Nothing happened. Month 2, i did the twitter DM. First had to provide all my information. Then my contract. To SHOW them that Agreement is for 5GB and not 4 GB. I give screenshots of all the monthly sms i get fr them as well as the statement where it still shows the contracted data and total. I am not getting joy fr their "investigation". Month 3. Still waiting and still giving them screenshots of what is received and what I pay for. That is R400 less than what they debit me every monthI have all the referance nrs.