In my case, Omega DII with stock 4x5 condensers on 6x6 neg. Wrong condenser set for the neg. Way bigger, covers the neg with light but...
In the DII there are different lens cones that put the 135mm lens far away from the neg. The condenser set that works for a lens that has to be far away from the neg is not aimed right for an 80mm lens (on a flat board) that has to be really close to the neg.
When I lifted the head to remove the negative (another DII 'feature'), I could see the falloff go away. The light improved. I could have held the head up and gotten even lighting. That's how I immediately knew the falloff was due to the condensers. Fortunately I had the correct set of condensers so I just swapped them out and continued.
I could swear I use the 80mm for 6x9 all the time with no coverage issues. Have to check my notes. This time I needed the extra contrast, otherwise I usually use a fluorescent head where none of this stuff about condensers applies. (With a DII anyway, it is painful because they are not adjustable).