It may be that you have not set the relationship options. You have not said what tables you have related so it is not easy to explain in terms of your system. So I will have to use an example.
Let's say you have a Person table and an Address table. So a person can have multiple addresses. The table occurrences in the relationship graph are simply linked:
Person::ID = Address::ID_person
What you will need is the ability to create related Address records from a Person record. So you go into the relationship graph and double-click the relationship box between the two table occurrences. This opens the relationship options. Down the bottom, on the Address side, you check the box to "allow creation of related records in this table".
Back in Browse mode, where you have created an Address portal on a Person layout, you will be able to enter an address for that person. This will create an Address record that is linked to the current Person record.