one unsharp corner on prints? Help!

Hello,
I need some help. I have been making some 8x10 prints from 4x5 slides using
my 150mm rodenstock rodagon lens.
For the most part my prints are full frame. I am getting one corner that is
not sharp and in fact it looks like movement. It always happens in the same
corner. I re-aligned my enlarger and assured my paper is laying flat. My
easel is totally flat. When I change to my 135mm Nikon lens problem goes
away. Prints made with my 80mm rodenstock lens from 6x6... no problem.
That I know of I haven't noticed this problem before with the 150mm lens.

Am I doing something wrong? Could it be the lens? Has anyone experienced
this before?

Thanks
Ed Margiewicz