There can be several reasons why this might happen, but the following troubleshooting methods should be able to help out quite a bit.
1) If you're using Paypal or a Debit Card, make sure there's enough money in the account that you're using for your eMastered subscription.
2) If your card number or info has recently been changed, make sure to change your billing info in the "My Account." section of the website. We have an explanation for how to do this in our knowledge base.
3) Trying a different device or web browser can also impact some scenarios, so please let us know if the issue continues when you try on a different computer/web browser as well.
4) If your card continues to be declined, you may still be able to use the PayPal payment method with the card.