Poor service by MTN South Africa

Last night I had 9 Gig data on my contract Sim which I use as my wife data. Just a few munites when I checked my balance it shows that I have now 480 mb. This is unsord and rediculous. I called their customer care line and the lady that Assisted me said she saw that I used less than 1 gig but doesn't know what happened to the rest of my data. I get 40 gig 20 gig all day and a 20 gig night owl. This is not the first time I encounter this.