Every now and again a Credit Card will get declined. Here is what to do.
Verify Credit Card data is correct. Its often easiest to remove the current card on file and add a new one. (can be the same one or different)
If that still doesn't solve the problem, get the member to ask their bank why the charge was declined. Banks typically don't provide much more data than a generic decline to Stripe so not a lot we can do our end without this additional information unfortunately.