Assuming it's a threaded headset, you may have tightened the top adjustable cup and locknut too much. It's a very fine line between too loose and too tight. If it's binding, you need to loosen the lock nut, then the top adjustable cup a hair, then with a wrench on the adjustable cup to prevent it from moving, tighten the lock nut. Make sure it moves freely without any indexing, but isn't so loose you feel any play.
