When a subscription payment defaults Stripe will automatically make 4 attempts at collecting payment and sends your client emails notifying them the card on file needs to be updated. After the 4th attempt at collection the client will lose access access to their subscription.
You can view clients who are past due from your dashboard by going to Reports > Subscriptions > Past due. You can also send an invoice manually by tapping the "past due" tag next to the client name:
I just transferred from another software and all of my subscriptions are not paid for this month!
You will want to remind them to enter payment methods if none transferred and/or complete the first months charge via invoice. Recurring payments will be automatic after this.
My Client has entered a card but they are still appearing as past due.
Go to your Stripe dashboard to check the details of why the transaction is not complete. Assuming the card on file is valid and not expired you will want to send the manual invoice from the reports page. Once paid, this will reset recurring charges going forward.