It's inevitable that there will be failed pledges at the end of your Kickstarter campaign. Kickstarter automaticaly alerts backers about the failed pledge and provides info on how to fix. You must take it a step further by messaging the backers and providing instructions. I also made a video that makes it easier for them to see how to do it.
Here's the message I send out to backers who have failed pledges. There are only 7 days for fixing the pledges before they are canceled. It's important to send out these messages at least every other day.
Your pledge for the [Marksmith Titanium Marker] needs some attention if you would like to receive it. The credit card on file with Kickstarter could not be processed. Please log back into Kickstarter to replace the card or retry the payment. It's likely that you will need to contact your credit card company to OK the charge.
You only have a few days to correct this issue, before your pledge gets automatically canceled by Kickstarter.
Take a look at this video about how to fix your pledge: https://youtu.be/T5bYf6r1Km0
More details provided by Kickstarter:
Log in to Kickstarter and click the “Fix payment” button in the banner at the top of your screen. This will guide you through the process of changing or retrying your payment method.
It may not necessarily be the case that there’s a specific issue with your card, so much as that you’ll need to ask your bank why this charge in particular is being declined. You may also want to include the details of this transaction, such as:
The charge being from Kickstarter.
The amount pledged for the project.
The date when Kickstarter attempted to collect your pledge. This was the date that the project you backed ended. If the project hasn't ended yet, you can use the date you attempted to pledge.
Additionally, it may be helpful to request that your bank “whitelist” any charges from Kickstarter.
Once you’ve contacted your bank, feel free to log in to the Kickstarter account from which you pledged and click the “Fix Payment” button in the “Important Notifications” banner.