Dunno.
Here’s what I did.
Loaded the tool from the tutorials disc.
Selected the sphere in the ‘sub tools’ menu
clicked on transparancy in the transform menu, and the sphere went transparent and I could see the skull drawing on the reference plane.
Edits work on the Sphere.
Let me just try it again… yep it works as I just described.
Are you using the ztool supplied on the disc, or creating an image plane/sphere yourself? If so, maybe that’s were you’re going wrong.
Sorry I can’t be any more help.
edit
Just tried setting it up manually myself using the plane, and sphere that comes with Zbrush, and it still works ok? Baffled.