Cannot purchase once off data bundles on contract

I have a my5gig contract and suddenly cannot purchase once off additional data bundles which I always been able to do and was advised I would be able to do when I took out the contract. Vodacom cannot suddenly change terms of contract. I tried to purchase online and although I got a message saying the purchase was successful, no bundle was activated. Upon calling the call centre, I was told that the system cancelled the order and they could not help me either as the option to purchase for my type of contract has disappeared. Seriously Vodacom, are you trying to lose customers?!? Maybe spend less time worrying about whatsapp and fix up your processes. Not only can i no longer purchase additional bundles but the amount of times your my vodacom website is not functioning is totally ridiculous.