Subscription renewal problem

I renewed my subscription for a year in June 2011 but my Norton Antivirus is now telling me it expired and I'm not covered. I received no confirmation email (as far as I can remember) but the order comes up on my Norton account. I've tried putting in the Product key to the subscription renewal key and it tells me it is invalid.  I'm sure I downloaded the product at the time I renewed and there is no download button next to my order in my Norton account. I only have a tenuous grasp on technology at the best of times and this is causing me a lot of grief. It appears I paid out for antivirus cover in June and haven't been covered since then!

I have just renewed my subscription for NIS to NIS2012 from NIS2011.

Why do I not get credit for the unused portion of my NIS2011 subscription?

(This is not the first year that this has happened when I have renewed to an identical package but one year on.)