Every now and again you will likely get a failed charge, whether its from a product purchase, event, recurring subscription, initiated automatically, from the Control Panel or person tried to sign up online remotely.
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.
Sometimes the same card will still work with other payment processors like Square but fail a transaction through Stripe same day (and vice versa). Very frustrating as it doesn't make much sense, but their bank is the best place to start.