missing columns on wfs (geoserver/mongodb)

we are currently checking up on geoserver over mongodb on our dev-environment. we are having some difficulties: we have a collection, containing different document which answer to the same structure ("schema-like"). when querying a wfs service from geoserver - some of the columns are missing. we discovered that if one of the "columns" on one of the documents contains null value - the whole column does not appear on the query response.

is there some configuration we are missing on the geoserver to return null values on wfs response?

Even if one instance of an attribute value equals null, the whole column is missing. I don't see how describefeaturetype may help as many q&a already suggested over the web.