Are you using the lens wide open? If so, you may want to try stopping down two stops which would put the lens at f/5.6 or f/8 depending on the lens.
Not familiar with the enlarger, but is the position of the lamp adjustable? Anything obstructing the path of the light inside the enlarger, or between the lens and baseboard? You can do a basic alignment check with any bubble level...make sure the negative stage, lens stage, and baseboard are parallel to one another.