First, contact your bank directly to resolve the reason for the billing payment being declined. (Note that is your bank that decides to approve or decline a payment.)
When successfully resolved, visit the "Settings" tab of your charity account and click on "Billing". Click on "Edit" below “Billing account” and create a new billing instruction by entering credit card details.
Charities registered in South Africa may also wish to switch to their SA bank account for billing purposes, under "Settings > Billing".