I was recently in a friend's darkroom. The friend had adjusted the three rotary dials on the colour head on the enlarger to zero (or so the friend thought), for the purposes of doing some tests. The results of the tests were very strange - almost no contrast in some cases.
After trying to figure out the cause of the problem, the solution finally presented itself.
The "yellow" rotary dial wasn't set to zero - it was instead set to maximum, which was far enough past the maximum setting as to appear to actually be set to zero.
So check the yellow filtration.
I won't identify the friend, because they post here .