This is something that bothers me in Notion. When you have a database with subitems, they never appear when I add a board view. But guess what? There are two workarounds:

1/ Filtering: You can add an advanced filter that checks all the conditions of sub/parent-items visibility at once. It's time-consuming and quite cumbersome.

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2/ Flattened list: Instead, you can just use the "flattened list" option in the subitems settings! It's way easier and nicer because it displays the related parent-item.