There sometimes is a delay in an itunes card getting registered after purchase. I had this problem once when I inputed the card a few minutes after the purchase. I was told that it sometimes can take a half hour before the purchase from a store gets registered with itunes, and sure enough after a short while it worked. It is not always like this as other times cards I purchased were accepted right away.
If this does not apply to you then I guess you could post an inquiry with itunes support here:
Where did you buy the card? Do you still have the receipt? For sure I would ask them about it!
TWO WEEKS! A half hour at most should have been more time than they needed.
Go to https://expresslane.apple.com/GetproductgroupList.action and let them know via the web your problem. I am sure it will get cleared up quickly.
Are itunes cards case sensitive? I never payed attention. Give that a try. It would be nice to have a bar code so you could scan the number right into a device that has the ability to use the scanning apps.