I have been using my own GG for a while now and never switched back to the old frosted GG. With my GG, I don't really have a sweet spot, I can move the camera away and to the GG and the image doesn't blur or go out of focus, but vignetting is present.
Well, I went back to the Frost GG for a test and I found something really weird - now I need a sweet spot. If the DV camera is not the right distance from the GG, it is out of focus - this is weird ! I wonder if it is because the GG I have originally spreads the light out more evenly and with a wider image, whereas the frosted GG has a much great hotspot and the image is smaller, therefore causing my mini DV to have the focus problems and need a sweet spot.
Any ideas?

the only thing I can think of is that you had your camera set on auto focus, and when you went back to the frosted GG, you switched to manual focus.

No, it wasn't that. I am just guessing here about the amount of information to focus on. Not sure exactly what it is...

When I use auto focus, the wax glass is really easy to focus but the ground glass is a bit harder, just don't know why.