A portal shows information from another RELATED table to the one you are currently looking at.
You need a relationship to make that true, based on a key field/value in the parent table and a foreign key field in the other. If the relationship is also ticked for allow creation of records, then you can click in the bottom empty portal row to create anew like record.
We figured this out.
The portal records were getting recorded. We could see them in the tables, but they would stop showing in the portal as soon as anything was clicked on.
The problem was that the Member Number fields that formed the relationship between the two tables and the main table were not set to Type: Number.