DEPLETION OF DATA WITHOUT BEING IN USE

I'm one of Vodacom customers for many years , lately I have been experiencing depletion of data after a few seconds. I have a contract with them, I receive 2GB of data both day and night. The nightly data disappeared without a trace without me using them. I then proceeded to buy an additional 1GB the following day my data was depleted. I have tried on several occasions to get the matter resolved but the agents are failing to give me answers and refuse to allow this matter to be investigated or discussed. I need assistance because I refuse to continue with this network if this is the kind of daily issues I come across.