When testing a subscription payment you need to remember that subscription is an ongoing payment so each email address can only have one active subscription for a particular item at a given time in the shop.
When you are buying the same product over and over again with the same email address it's not treating you as a different customer because in reality you are not.
You need to simulate a real world scenario whereby different customers with different email addresses will buy products. So when testing, make sure to delete your record from the "Customers Database" after you make a purchase by going to the "Manage Customers" menu of WP eStore plugin. This way when you test it again it will treat it as a new customer or use a different test product or paypal account to test.