When you are searching for your ancestors, it’s tempting to stick to searching for their name in an online index. But only a small fraction of records have been indexed. A larger part is available online as images that have not been indexed yet. These images are often available via the catalog or finding aids on the website of archives. Look for terms like inventarissen or archieven.
In some cases, the records have not been scanned yet, but the catalog or finding aid has a button to order scans, often for free. See the list of archives that provide free scanning-on-demand.