Membership Plugin Not allocate subscription

Hi Team,

I believe many have encountered this problem before, yet I haven't found a sensible answer.

In short, I'm use membership plugin to accept serial payments from paypal express. This process works fine except member is not assigned to any subscription after successful payment. I'm running single site with paypal sandbox. Notifications inside paypal are logged fine, but nothing appears in the "recent transactions" session in the paypal express payment gateway tab in membership. They however do show up in "pending transactions", with "Last transaction is pending." and Reason is set to blank. Any chance why is it happening?

P.S. I have IPN enabled with notification url set to http://mydomain/paymentreturn/paypalexpress/ (although other threads suggest it's not necessary).

Many thanks,