bad service delivery and customer service

I have a contract with MTN the flexi 220 and been getting airtime back for the value of 220 however 3 months ago it just stop and the airtime never appeared again so i had to call them to asked what the reason was no one could ever give a reason the only thing they would say was that they would escalate this to the necessary department and i will need to call back in a few days this went on for more then a month a week ago i called again same answer call back in a few days so at the end of the month debit went off then i got my airtime reimbur*****t but my problem is what about the months i did not get my airtime where did that go cause im paying for a service im not being delivered and MTN doesn't even have the decency to send you an apology sms stating what was the problem they are really bad when it comes to service delivery and customer service just wish my contract with them can finish now.