I tried this technique and got the same exact issue you're talking about.Seems to me the clay underneath the eyeball can't penetrate it for some reason.
@chrisradsby: My intention was to create a sword made of a giant bone but the base is made of metal. But maybe the bone is looking like a little bit clay:(
Using a mixture of the Clay build up, Damien Standard brushes as well as masking and inflating for the fingernails I have sculpted in a basic hand shape.