A couple of questions that might help diagnose what's going on: Does the image get unsharp abruptly, or does it "fade" from sharp to fuzzy. Is the edge of the fuzzy area parallel with the edge of the film, or does it curve?
You definitely can have a problem that's related to only one lens, though I'm having a hard time visualizing what sort of problem would lead to the effect that you're seeing. If the rear element of the lens doesn't protrude past the rear barrel, set the lens down on a table and check to make sure that everything that should be parallel with the table is parallel. This would include the lens board, the back and front of the shutter, and the top of the front lens barrel. If you're comfortable doing so, you might want to check the lens cells to make sure they're not cross-threaded in the shutter and that they're screwed all of the way in. (Just hand tight...they don't need to be screwed down hard.) Don't do any of this if you're not comfortable handling your lens these ways.
You mentioned in your original post that the lens board can be moved "in line with the camera". By this, do you mean that you can move it in the same direction that the bellows fold up? If so, you might try folding up a bit of paper and sticking it in between whatever locking mechanism your camera uses for lens boards and the board itself. That should hold the lens board tight so that you can see if that's the problem.
This is a stumper. I hope you figure it out soon...I'm sure it's frustrating.
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