I have an older 23C II enlarger with condenser head and El-Nikkor 50mm f/2.8 lens. Sharpness isn't a problem for me with the lens wide open at f/2.8. If your 23C can't accomplish 35mm full-frame sharpness at *any* available aperture, the lens could be the culprit.
Finally, I don't remember if you said that the lens was clean or not. There aren't any smudges or anything like that around the edges are there?