Plural forms with parsetree and search in Dutch

[hekl]

I am using parsetree(text, lemmata=True) (pattern.nl) and search(searchterm, text) for Dutch.
Like: search: "baan" in "Het aantal banen in Noord-Holland neemt toe."
This has the advantage of constructing a lemma and also searching for a plural form of the search term. But not in all cases. "Stad" as a search term also searches for "steden". But "baan" does not search for "banen", "arbeider" does not search for "arbeiders".
Is there a logic to this? The pluralize function gives the correct plural forms.