Donations can fail for a number of reasons. A donor's card can have insufficient funds, their card might have expired, or maybe it was for another reason. This is most often an issue when a donor has set-up a recurring donations and one of those instances causes their scheduled donation to fail. When this happens, you have the ability to login to your dashboard and Retry the failed donation.
To Retry a failed donation, click Giving > Donations, then find the donation in question and click View...
Once you have the Donation opened, click the menu button in the right corner of the screen and select Retry.
Once a donation retry is successful, a new donation is created for the successful transaction and the old failed donation remains for historical purposes.
Tip: To avoid creating an uncomfortable situation with your donors, but sure to reach out to them before retrying their failed donation. Or better yet, you might wait for them to reach out to you about it.