Disappearing Vodacom data

I have a Wifi router data contract with vodacom for the past 5 or six months my data just disappears and and when I call to complain they cant explain instead just offer me free data and the problm will stop the following month and come back back again recently they allege i used 20gig of data in 6 days but cant explain how and they offered me free 10gig data only to loose it again the following day i am 101% convinced VODACOM just drains my data to have me spend more by buying data I am not an excessive data user I am able to use the 20gig day data for a full month but lately my life is hell as I am still stuck paying for this contract with data I hardly use it just disappears