Airtime and data keeps disappearing

My data and airtime keep disappearing with no notification of data depleting. I called vodacom last week Thursday because I had no data and airtime that morning, and I did not know why. I had received NO sms informing me how far my data was not even an sms to say its depleting. The consultant said she will log a call with the technicians and that I will receive a call from them to investigate in 48-72 hours. I then recharged airtime and topped up data with 1gig. I did not reveive a call from a technician. And again, yesterday, Tuesday 5 September in the afternoon (5 days later), with NO notification of data depleting, I sit with no data or airtime. Please explain why my data keeps disappearing with no notification and how data that I use for longer periods of time can just vanish like that in a matter of days. Oh and by the way, I use wifi MOST of the time!