Data disappearing

I would just like to know how 6GB data can just disappear in a week? I got my 6GB on last week Sunday and today a week later it is just gone! I have WiFi at home and is almost always home. So how is that possible? Happens last year around this time as well and if you ask MTN they just say they don’t know. Easy answer. I am not the only one complaining about this. I know a few people that have the same problem. I paid for that data that just mysteriously disappeared. Sick and tired of this now.