ref: https://stackoverflow.com/a/25730860/2735734
How to Tell PayPal to automatically process the monthly payment?
here is the PayPal Reponses
Once the user completes and submits subscription purchase page you have created, the subscription will begin and will automatically rebill the customer's account on whatever interval you have selected. You do not have to do anything.
Updated: The REST API also automatically processes subscriptions. You do not have to resubmit monthly. The PayPal sandbox was updated in July of this year to enable you to test and process subscriptions. There are no issues being reported in the dev community about this not working, so I suspect it is something on your side.
If you haven't already. please review this:
https://developer.paypal.com/docs/integration/direct/test-the-api/
There are billing plan and billing subscription sections. If you still have a problem, post your billing plan code and the response you are receiving.