Like Gary said...I was starting to wonder if there was some inadvertent focussing of the lamp due to an unfortunate combination of enlarger component positioning.
I recently started looking for reading material on point source enlarger light sources, and apparently it is common to get 'burn spots' when the lamp isn't where it should be...unfortunately I don't know what the proper combination is that eliminates the phenomenon.
Now that I think about this a bit more, is it possible the lamp was actually focussed on the negative instead of spread out to cover the neg. properly? This would be another way of saying wrong lamp position relative to the optics.
These are 'armchair' suggestions, just ideas to examine.