It's important to remember that RaiseDonors is not the one performing these charges. RaiseDonors never actually interacts with your bank of the customers credit card for PCI compliance reasons.
In this situation, you have selected Stripe to be your merchant account - and they are the ones who charge your donors credit cards.
Here is an excerpt taken from the Stripe website regarding declined charges.
Why was one of my charges declined?
Declines can happen for a variety of reasons. When we [Stripe] submit a charge to your customer’s bank, they have automated systems that determine whether or not to accept the charge. These systems take various signals into account, such as your customer’s spending habits, account balance, and card information like the expiration date and CVC.
Since these signals are constantly changing, a previously successful card might be declined in the future. Even if all of the card information is correct, and your customer previously had a successful payment, a future charge can still be declined by a bank’s overzealous fraud systems.
How can I find out more information about a specific decline?
We [Stripe] show as much information as we [Stripe] receive from your customer’s bank about a decline. Unfortunately most declines are generic, so we [Stripe] don’t have much information as to why a charge was specifically declined.
If all of the card information seems correct, it is best to have your customer contact his or her bank, inquire for more information, and ask for future charges to be accepted.
How do I decrease the likelihood of declines?
The correctness of the card number, the expiration data, and the CVC are the primary factors used by the customer’s bank when deciding whether or not to accept a transaction.
Collecting the CVC can significantly improve your decline rates. If you’re not collecting CVCs and you’re having issues with declines, requiring the value can be a quick fix.
The influence of other data that you collect, like the address or name, varies by card brand. For example, only American Express consider the customer’s name. If you are still seeing troubles with declines after collecting the more influential fields, it might be worthwhile to collect this additional data.