Losing days of NS when upgrading

Norton told me I have 21 days of subscription left. I bought a new NS and put in my new product key, as prompted.  Now it tells me I have 366 days - the same amount as if I had zero days remaining on my previous purchase of Norton Security.  The software prompts did not give any warning that days would be lost from my old product key if I switch to a new one.  What can I do to get those 21 days added to the new key? Thanks!