I'd say it's the lens.  But just to be sure set up a negative and do a print
that shows the problem.  Don't touch the negative and rotate the lens 90 deg in
its mount.  If the unsharp area moves to follow the lens rotation you've proven
it's the lens (but could also be the lens mount board if you're using something
like a Beseler.

Curtis

Thanks Curtis and Robert I am going to try both.
Ed

Try inserting a sheet of glass with some lines drawn on it or a sheet
of ground glass above the carrier in place of a negative.
Focus on that and see if everything looks sharp. I once had an enlarging
lens with the same problem.
I would think alignment was more likely, though.

Quite likely the carrier is not centered with the lens and you are
running the image outside the circle of sharpness of the lens.
Remember the circle of illumination and the circle of sharpness is not
the same. Most lenses lose definition when approaching the limits of
thier circle of illumination.
Does the corner sharpen up by much when the lens is stopped down to
f/22?
Possibly the lensboard has an off center lens hole in it.

The importance of lens alignment dawned on me about thirty years after
I started printing.  The center of the neg and the center of the lens
need to be aligned.  They need to be on center and they need to be
parallel.

There are lots of methods to doing this.  Among the current popular
methods is to use a laser.  You can make a suitable laser out of one
of the cheap, small laser levels from the hardware store and a penta
prism or right angle prism.

Here's the method I use.

1.   Level the easel and place the laser on it so the beam is straight
up.  (That's the hard part.)

2.  Put a good, first surface mirror in the negative stage, and make
sure it is parallel to the easel (no lens installed).  Repeat this for
the lens stage.

3.  Install the lens and position the laser so the light is aimed near
the center of the lens.  Adjust the horizontal position of the
negative stage and lens to make sure the center of the negative stage
opening lines up with the center of the lens.  (Laser, lens, and
negative stage are co-linear.)

4.  Put the mirror back in the negative stage.  If everything is in
alignment, the laser light should go to the negative stage and back
along the same path -- even if the lens is slightly off center.  It
took me a while to realize this, but if the laser light hits the lens
straight on, it should leave the lens straight (at least close to the
center of the lens).  If the laser beam hits the mirror straight, it
should be returned along the same path.

Hope this helps.