Data on contract incorrect

I have noticed for a while now that by the 6-8 of the month that my data bundle on my contract is already depleted. I contacted the Call Center after noticing that my data bundle was only 200Mb, but, no joy. After explaining to the Call Center Agent that when I took the contract, the particular bundle on the contract that I wanted was not sufficient being in IT and working all hours. The Sales Agent that helped me at THAT time changed the contract to include extra data at a slightly higher cost, and would bring the 200Mb data bundle up to 800Mb, which I agreed to. It was never a problem until recently when the discovery was made. Needless to say, very very disappointed in Vodacom. Probably will NOT be renewing my contract with them. Catch 22 situation, as the SP's are all as bad as eachother.