Transfer for Subscription does not appear on my account in Sandbox Mode

[todorcs]

I've integrated the Subscription API to my Application as per the documentation at https://developer.paypal.com/docs/subscriptions/integrate/ and it seems to work since I get a response from the '/v1/billing/subscriptions' endpoint:  

however, the transactions do not appear in either of the seller or buyer accounts, so I'm kind of confused.

Can somebody please point out what can cause such an issue?

Thanks in advance!