This is the 3rd time in 5 months my data missing

hi guys, i have no words to explain that i lost 1.4 gig average in day though i didn't used more than 30 - 50mb during whole day and most of the day my phone connected to wifi at home, i am shocked and disgusted with vodacom to know that they doing a data *****ing, i am struggling understand as well why is it happening to me again and again, can you guys please explain why is it happening and is there way that you guys can fix it for me.....pls vodacom dont ***** my hard earning money