DATA & SMS BUNDLES

I have a Smart Light contract with Vodacom.I pay for 55 free minutes,100MB data & 100 sms pm.At the end of every month the balance of my data & sms just disappears.How can Vodacom take services that I've paid for, back?Why is the balance not carried over to the next month?If I go to a grocer.Buy R1000 worth of groceries. At the end of the month they don't take back the groceries that I've not used! THIS IS WHAT YOU DO!I've phoned your call centre.They've logged an enquiry to the tech dept.They told me that the tech dept will contact me.Up to date, I've not received any feedback.If I use my data and sms sparingly because I know that I will need them the next month, how can you just let them expire at the end of each month?I'VE ALREADY PAID FOR THEM!I've spoken to a lot of my friends.Some of them have the same problem, but a lot of people's unused data and sms are carried over.WHY DOES MINE EXPIRE EVERY MONTH?Vodacom can you please solve this problem. I really do not want to pay for stuff if you take it back if I have not used it in a month's time. I'm not asking anything extra, I'm just asking to use the services that I've paid for. Thank you for your assistance!