Remove air bubbles from the tip of the buret and the stopcock by draining several milliliters of titrant into a waste beaker.
If the bubbles persist, try rapidly running the titrant thorugh the buret tip. Then grasp the buret and give it a bit of a downward jerk while the stopcock is open.
IMAGE. Close-up of the buret tip. Note the large air bubble that must be removed.
Video. Removing air bubbles from the buret tip.