Subscription not assigned after signup/payment

This is for Membership Premium.

We had a user sign up for a yearly plan and they made the payment through PayPal however they weren't assigned the correct Membership Level and associated access to our premium content.

How can we fix this? Is this a PayPal IPN thing or something else?

When people pay close to $3000 for a plan, we need to make sure they have access right away.

Thanks!