There are subscriptions created in Paypal, several years ago, which are paid monthly.
I configured a webhook (PAYMENT.SALE.COMPLETED) to receive notifications of these payments, and so far I have not received any notification, although payments have already occurred after the configuration.
There is any Paypal limitation to receive webhooks related with previous subscriptions?
I'm able to receive notifications for Single Payments, with the same webhook Also, I checked on Paypal Backoffice and there is no error trying to send the notification
Determine which client-id/app is being used for the subscriptions, which may not be the client-id you expect. Then locate that app within https://developer.paypal.com/dashboard/applications
That is the app where you need to register a webhook listener, ones registered for a different app will not receive events for it.
If the subscriptions were definitively not created using the REST API (which includes the JS SDK with createSubscription) but are rather older subscriptions of some type (for example HTML subscribe buttons with no JS), then you need to add an "NVP/SOAP" webhook listener in that same applications view (Live mode). Look for it at the bottom of the page listing your live apps.