Michael, the jump probably means illumination onto the Stouffer was uneven. I suspect the right half of the chart got more light than the left part. If you're using an enlarger as a light-source, this could mean the film-holder was not centred under the enlarger, so that the left half was farther away from the centre, experiencing fall-off from the enlarger's lens. Or the light-output of the enlarger might be uneven.
Originally Posted by Michael Hoth
I suggest checking centring. You also might want to use an enlarger lens with a longer focal length and raise the head some. An easy way to test uniformity is to expose a piece of film in your film-holder, with *no* Stouffer present. Then measure density at various locations across the negative.