Membership account not being created with subscription

This has happened twice on our site. Someone signs up for a subscription membership, is taken to PayPal and pays for the subscription.

We get the notice from PayPal that the subscription has been created and the payment went through.

No Membership account is created - they are not listed as a WP User or in Membership > Members. Thus we have to then create their account manually.

Before the site launch we did extensive testing with $1 subscription accounts and these were working with Membership accounts being created.

What could be causing accounts to not be created? PayPal IPN/some other PayPal setting? Something else?

We can't exactly throw up test subscriptions on a live site so my next step will be to port this to a test domain so we can troubleshoot further but any help would be appreciated.