Hello Angelo, thank for your answer.
At last the problem was related to geoserver-2.14.1, converting from SHP to GeoJSON.
Following the tutorial at
http://www.aiknow.info/2018/07/23/knowage-location-intelligence/, during the export from the "Layer Preview tool", by default GeoServer limit the number of features inside the GeoJSON file.
I've to remove the "features" attribute from the generated export URL (or to set it to a very high value), so the generated GeoJSON file was bigger and will contain all the data and features needed.
You can consider the issue closed.
All the best,
Alessio