Is anyone else having issues processing a newly added credit card to an existing customers account?

Hello there, heidi46. 

May I know what specific issue you encountered? It will help me accurately address your concern.

When adding new credit card information, Intuit/ QuickBooks Online (QBO) will perform a quick validation. In the event the validation fails, you will receive a message that the card is unable to be saved at this time. You can request another form of payment from your customer. Here's how:
 

1. Go to Customers & leads, then select Customers.
2. Choose the customer you want to edit.
3. Select Edit in the upper right. ( If you can't see the Edit button, you need to set up the credit card payment methods in the Payment List.)
4. Find the Payments section.
5. Select the current card number below the Preferred payment method line.
6. Select the + Add link in the upper right.
7. Enter the new card information, then select Save.

You can also check out this article to keep track of all your credit card transactions: Download the most recent bank and credit card transactions in QuickBooks Online (intuit.com).

Please comment below if you have further concerns about adding a new credit card to an existing customer's account. I'll be here to assist you. Take care!