Family Sharing Credit Card works for Organizer, not for Spouse

My wife set up our Family Sharing Plan and added me as the Trusted Adult. However, the credit card she has set up works fine for her (she can purchase an App) but not me. I hit the "Purchase" button in the App Store, use my Touch ID, get the blue Check Mark, and then a pop up notice that says "Payment Declined." The card is fine. She used it to buy the App for me (since we share apps). But this is annoying. Any ideas?