Newly registered member never get activated even after pay through paypal

I setup the membership plugin with only one Paid membership subscription. I tested by using the registration form from the plugin. The user is created with an inactive state but still redirect me to Subscription page, which show the Paypal button link to pay. I proceed to pay through Paypal Sandbox and the transaction go through without problem. (Both seller and buyer sandbox has the transaction record in their account) However, when I go back to the WordPress site, no membership detail is being updated. The user is still inactive and not in any subscription. I am just testing it locally, will that be a problem?