1. Be sure the Drill is the correct diameter.
2. Check the Drill Point to be sure it is sharp and not damaged.
3. Check the Drill to be sure it is not bent.
4. Check the Drill Collet to be sure it is clean and not not broken or damaged.
5. Be sure the length of the Drill Point cutting edges are the same.
6. Be sure the Tool Holder is properly aligned with the centerline of the spindle.
7. Make sure the Feedrate is not too high causing the Drill to deflect or bend.