Specific error message?
This article is usually helpful --> http://support.apple.com/kb/TS1292
And ensure you are redeeming it using the proper procedure --> http://support.apple.com/kb/HT1574
If you still cant get it, please contact Express Lane at http://expresslane.apple.com and submit a request for the iTunes Store to contact you to resolve the issue. Be sure to submit your request under iTunes > iTunes Store regarding the issues you are experiencing with the gift card. They will respond via email within 24 hours.
I hope this is helpful!