If a folder shows no emails although there should be some (e.g. after an import), a view filter is often active. You can tell by “Filter applied” in the bottom left of the status bar.
Steps
Select the affected folder.
In the bottom left of the status bar, click Filter applied.
In the window, click Clear All and confirm with OK.
Tip: If there's no message in the status bar, choose View › Reset View. This resets all filters and sorting for the folder.