I just saw this thread, hope I'm not too late to post -
I have continuing problems with NR, in contact printing (with an enlarger, with a piece of foam and glass, enlarger up at about 11x14 size on enlarging paper.) All from 4x5 negs.
I have tried:
1/4" plate glass, window glass, AntiGlare glass, with the enlarger lens in focus, way out of focus, with no lens (raw enlarge light), with a layer of diffusion material on top of the glass, winter, summer (different humidity levels). I even tried the Ed Weston method - ceiling light exposed manually. I have not made one print without NR's. They can appear in any tonal area, and I can see them during exposure. I finally gave up. Any suggestions would be welcomed.
BTW - In the 70's I did a lot of contacting (the same way, with the enlarger and 1/4" plate glass) with never a problem, in a different darkroom in a different part of Virginia. The only difference that I can think of was that that darkroom (in a university) was perpetually air conditioned (or heated). My new darkroom is in my basement, but I have a dehumidifier going all the time (50%RH).