1. Check the drill point to see if the drill is dull, broken or damaged.
2. Check the Drill Collet to be sure it is holding the Drill securely.
3. If all holes created by this Drill have the same depth problem, check the Z axis Geometry and Wear Offsets for the Drill to be sure they are correct. Adjust as required.
4. If the other holes created by this drill are correct in depth, adjust the Z axis coordinate in the Program of the problem hole.