The last four rolls of FP 120 that I developed in a 4 roll nikkor type tank were a letdown. I use rodinal 1:50 for 18 minutes. I know the film was loaded properly, but thru each frame when hung vertically (not the between frame area) the middle one third is lighter than the sides, and it is the same in each frame. Other rolls taken with the same camera did not show similar problems. I aggitate for the first minute followed by 10 seconds each minute. I don't frequently use a 4 roll tank, prefering the 2 roll tank. Does the tank make a difference? Is the agitation correct? Any other ideas on what could be wrong?