Containers can either be created for a specific Sample or a Material.
The system is designed in such a way that if a user chooses to make a container from the Samples table where a Material ID is linked (i.e. the Material ID cell has a value), the container will be created under the Material (with batch if applicable). It will not appear in the Samples table since the container is associated with the Material and not the Sample.
If the sample is not linked to any material ((i.e. the Material ID cell is blank), then the container that you add get displayed under the Containers column in the Samples Table.
You can find this documented in the Help Center - Signals Notebook > Working with Information Items > Working with Samples > Creating Containers from the Samples Table