Can you tell us what enlarger you are using?
Negative size?
Can you also scan your image so we can see it?

Couple of ideas....

Are you using the correct condenser for your film and lens type? I can see this being a problem if you are cropping your image.
If you are using a type of enlarger with movable condensers, is it in the right place?
If you are using under-the-lens type contrast filter, is it centered with the center of your lens?
Are you sure nothing is casting a shadow?
Are you sure your lamp in the enlarger is centered correctly?
Did you remember to feed your cat?

It's all random thoughts because we know very little of your equipment and process.