I'd be concerned if it were more than 1/10 or 1/5 stop. You should be measuring with the lens focused and with the negative carrier in place.
If it is more than 1/5 stop, be sure you have the right condenser for the format (or for a larger format), that it is in the correct position, and that you have an appropriate focal length enlarging lens. If all that checks out, try stopping down the lens and seeing if things improve and take note of the widest aperture you can use without significant falloff.