I have a small darkroom where I've just started making B&W prints from 4x5 negatives. Even when the negative is very in focus, and scans nicely, when I print 8x10 with my enlarger the details aren't very sharp. I use a smaller enlarging lens aperture (f11 - f22) and a grain focuser. The paper is Ilford MG RC Glossy, the developer is ilford multigrade developer. Paper exposure tends to be at least 16 seconds, time in developer about 2 minutes. Enlarging lens is a Wollensack 135mm f/4.5.
What could be the cause of this loss of sharpness?