Auto Light table broken in PC 2.5

Howdy
When I turn on the Auto Light Table in Toon Boom 2.5 ( PC) all the layers are fully visible rather than greyed out. This makes adding things to busy layouts very confusing. The Manual shows an image of what should happen, with drawings on the selected element clearly discernable from drawings on the other elements but it does not appear to be working for me. I’m using an Nvidia GeForce4 graphics card and I have this problem in both OpenGL and Direct 3D modes. It worked fine in version 2
Does anyone know how to fix this?
Thanks
Kengo