WebService - Not All Properties Showing On Client

I have a vb.net baseclass in a dll in my c# project. I created a derived class in c#. I fill the C# class with all its properties, including the base properties from the vb class. Now I send them through a webservice (c# ) to a jQuery client. But on the client I only see the vb properties?

public class FilmItem : ContentItem // ContentItem is from VB DLL {
public string Zender { get; set; }
public string Jaar { get; set; }
[WebMethod]
public IEnumerable GetContentItems(VBLib.GridRequest gridRequest) {
ContentCache contentCache = new ContentCache();
return contentCache.GetFilms(gridRequest); // gives back a List<FilmItem>
}

Just found out that if I remove the vb.net class, all the properties are showing fine from the C# class. Also found out that the vb.net is declared like this:
Public Class ContentItem
Inherits System.Collections.Generic.Dictionary(Of String, Object)
Implements IContentItem
Could it be the dictionary or the interface? (Which sums up the properties I was seeing).