If your client's payment fails, the system will still try to process the payment a couple of more times, during that period you will see the invoice status as Retrying. If this is happening, you can share a payment link with them so they can update their payment method directly from there, here's how to do it. They can also just update it from their profile.
After the system retries a couple of times, the invoice status will change to Unpaid if there's still a problem. You can check the invoice status of a client by going to Clients, finding the client you want to check and then clicking on the Invoice tab.