1. Dies may be out-of-match.
2. Dies may have too many shims under dies. Remove and replace with a single plate.
3. Blanks may be tipping at the Start End. Adjust the air assists, increase Hold-Down finger pressure or grind additional clearance to fit any shoulders on the Starter Finger.
4. Check for slipping of the blanks at the Start End. Replace worn dies. If not worn, increase start end squeeze or use Corrugated dies.
5. Be sure the Die Relief surface is smooth and does not have an uneven surface.
6. Make sure the Dies are correctly seated in the Die Pockets. If not, the helix angle can be affected causing drunken threads.
7. Check the dies for the accuracy of their thread form and replace as required.