Hm, could you please check that the KML to Layer works fine first then?
1) Use the Layer to KML tool to export any of your shapefiles/feature classes (with multiple fields) into a KML/KMZ file.
2) Use the KML to Layer tool to export the newly created KML/KMZ file into a feature class.
Do you get all your fields then? By this I am trying to verify that the tool itself works correctly. And if it does, then there is probably something wrong with the data you used.
Do you have ArcGIS 10 with the latest SP installed (SP4)?