If a customer contacts you saying they have a paying subscription but they are unable to access content, it is usually due to them having multiple accounts.
To investigate, head to your Subscribers tab and search for the customer by name (or search by the email they have provided and then use that to search by name). You will usually find the details of the duplicate accounts.
You can delete accounts they no longer need, and they are also able to change their email directly in their account.