No Fisheye Effect when zoomed in?

I bought a Fisheye Macro adapter for my Nikon D5000, and unless i'm zoomed out at 18mm, the "ring" and fisheye effect is no longer noticeable above that 18mm.

Should I add another macro filter to see if this changes?

Fisheye adapter here:
http://www.amazon.com/...0017W70JE/

Thanks!

Added (1). Note that I can't afford to spend $900 on a real fisheye lens, thanks for the effort though…

Alternative is buying a real fisheye lens

This is why you should use a real fisheye lens with a DSLR instead of a cheap adapter. You get what you pay for…

The fisheye adapter is really a supplemental diopeter to make your lens a much shorter focal length. The fisheye effect BY DESIGN will be most pronounced at the shortest focal length of your camera lens.

"Ye canna change the laws of physics, Captain!"--Montgomery Scott, Cmdr., Engineering Officer, USS Enterprise, NCC-1701

There's nothing wrong with the adapter, this is how they work.

This is why a fisheye lens (a real one) is almost always a prime lens.

Image quality is already taking a hit from the adapter, adding another will just make your pictures look like they were taken through coke bottle lenses.

Well that's the way it's designed. Why would you want a fisheye effect when you're zoomed in?

Alternatively, just add a fisheye effect when you edit.