Double adapting lenses = quality/focus issue?

So, I have two adapters. Pentacon Six -> Mamiya 645 and a Mamiya 645 -> Nikon. When I adapt the Mamiya 645 to Nikon the focus seems to be alright, but when i double adapt a P6 lens to Nikon, shooting wide open and getting a in focus shot is not possible. I didn't do a tripod test, but the focus did look right with the split focus screen.

Thoughts? Double adaption of lenses = bad? P6 -> M645 russian junk?

Those adapters should just be mechanical tubes without any optics, so that shouldn't be a problem, as long as the tubes are of the right length to allow normal infinity focus (bearing in mind that your lens might focus past infinity).

Do some tests with a tripod, focusing carefully, and seeing how it works out.

It might just be that the Pentacon lenses don't hold up to magnification as well as lenses designed for 35mm. Try stopping down to around f:8-11, which is likely to be the sweet spot for best compromise between lens aberrations vs. diffraction. When I had a Pentacon 6, the best lens of the four that I had was the 50mm Flektagon, and there would be no reason to use that on a Nikon, since you probably already have a faster, better, Nikon lens of 50mm designed for the format. So you must be using a longer lens, and a longer lens requires a faster shutter speed for handheld use or a tripod, so that can also be an issue.

I've tested it with a Vega 90mm a while back and was displeased then on a smaller format, but when I tried my new Zeiss Jena 80mm, I was shocked. This lens does not focus past infinity. I'll try a better test later.