You're encountering vignetting. This can be caused by the lens, the condensers, or various other things. My own Nikon EL-Nikkor f/2.8 lens doesn't seem to vignette much, but I also seldom print at f/2.8. My own times are usually in the 10-40s range for an 8x10 print at f/5.6 or f/8. An 11x14 would require about twice that, or open up by one stop. My enlarger is a Philips PCS130 with PCS150 color controller.
Thus, I've got a few suggestions:
- Examine your lens for signs of damage -- dirty elements or fungus, say.
- Examine your condensers in the same way.
- Verify that you've got the right condensers in place, and that they're positioned correctly. Condenser enlargers typically require either swapping condensers or shifting their positions for different film formats, and if you've got yours set up wrong, you could get vignetting.
- Check your bulb. You might have a bulb that's dim from clouding or that's of too low a wattage. Changing the bulb might not affect your vignetting problem, but it might shorten your printing times. Don't use a bulb of a higher wattage than the manufacturer recommends, though.