Everytime i load airtime it dissapears

Hi,<br> <br> I have been loading airtime on my cellphone and it just disappears. last month in August i cant remember the date o loaded R60 airtime and i received an sms from mtn saying \your account has been loaded with R60 airtime. when i tried to make a call it said "you have insufficient airtime to make a call\"". i checked my balance and it was 00. i called mtn and they said they will investigate the matter and come back to me in 48 hours. It was a week"