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The purpose of a neutral density filter is to let less light through the lens. That doesn't mean that the photos will come out any darker, any more than photos taken indoors by natural room lighting come out darker than those taken on a sunny beach at midday. The art of exposure is to get the right image density, and your meter is trying to do that. And succeeding, as you can't see a difference.
The reason for using a neutral denisty filter is to allow you to get the correct exposure, but use either a wider aperture (for less depth of field) or a longer shutter speed (to remove people from scenes or blur things).
If you are manually stopping down your lenses, you're probably coming up against the problem that autofocus doesn't work at small apertures - typically f/5.6. I'm assuming that the lenses are stopped down, and not relying on the lens closing down when you make the exposure.
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