Inability to use device paid for

I have a wifi device and for a few months now, my sim gets locked before monthend and I cannot use my data... even if I have sufficient data available. When I logged a call with customer service last month, I was told that I have reached my credit limit so the sim is automatically locked. The consultant could not clearly explain how my credit limit was reached and how to avoid this. I am very unhappy with vodacom and this contract. I end up using my MTN sim to access the internet and it WORKS!! I am nearing the end of my contract and I will not be renewing it with vodacom or taking up another vodacom contract.