I have never used that paper, but a generic cause of this sort of problem might be an aperture that was not closing as expected or a bulb/lighting-system that was over powered ??
Workarounds might include using a less powerful bulb (not handy in a teaching-darkroom), using a VC filter (if not using one currently), using an ND filter on the lens or in the filter pack, switching in the neutral-density filter in the dichroic head if one is fitted, saving the paper for high degrees of enlargement where the times would otherwise be rather long.