It is likely that you are using a "Relate" field rather than a relationship that exists between the two modules. A Relate field created in studio is, simply put, a link to the other record.
You need to create a relationship in studio, for which there are two kinds
One to Many relationship - An example of this would be the standard Accounts to Opportunities relationship. On the Opportunity record there is an "Account" field, but on the Accounts module is an Opportunity subpanel (As an Account can have multiple Opportunities)
Many to Many relationship - This relationship would result in subpanels on either module.