I would contact support as it may be an issue with the code or something on your account preventing it.
You can contact through requesting a call using the following link:
Or by calling 1-800-4-MY-XBOX
Support can look up the code and make sure its activated, it could also be a problem with the region for the code and the region on your account not matching
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the points may not have been activated and support is going to tell you to take the card back to the store and have them "activate" them or get new ones
The first thing you should try and do is take the card back to the store from where you bought it and ask them to activate it again.
If that doesn't work contact Xbox LIVE support using this link;
They can see if it's activated, the problem might also be that there is a decline or a mistake in your billing information so I suggest you double check it;