To try and get your issue resolved as quickly as possible, try the following troubleshooting methods first:
Have you been able to delete and redownload the track in question?
Did you have a steady internet connection while downloading the track?
If you answered yes to both questions and are still having an issue with your DLC, you may need to reach out to Nintendo Customer Services in order to receive an account adjustment. Their support portal is at the following address:
In the meantime, if you can provide us with the following information it will greatly help us investigate the issue: