Re-read Mick's post.

Are you saying the prints are out of focus,
or are you judging the image in the magnifier ?

If you focus the image with the Peak,
and the prints are out of focus,
then all you have to do is confirm that you've adjusted the eyepeice properly
(look at the directions packed with the Peak). This IS important.

'Focus Shift' is a design flaw, not some sort of entropy
like a toaster evolving into a lawn mower.

When you stop down, you're have four problems;
depth of field, a 3 dimensional subject, and diffraction.

Diffraction reduces the resolution of the lens (as magnified by the Peak).

AS you stop down, you are now looking at an image with depth,
as the increased depth of field makes it harder to discern the exact plane of focus.

Finally, the image is darker, harder to see.

If the pictures focussed at f/5.6 are not good, and the Peak's reticle is adjusted,
then the lens might be suspect. But there isn't much that ever goes wrong with an enlarging lens.