I suspect you neglected to take into account that lenses used at this magnification have a bellows factor to be considered. Most people only think of this for large format cameras,but it does come into play on the Mamiya twin lens cameras when the bellows is racked out for close ups.
Using this factor your effective aperture is somewhere around f32-f45 (I'm not in the mood to do the math work before breakfast). This would cause not only the under exposure to which you refer, but also is more likely to get you into the realm of diffraction.
There is a scale on the side of the Mamiya to help you compensate for the bellows.