Double flaring SS as your finding is a bit different. Five things I found go a long ways towards solving the issue.
1. You must use annealed tubing.
2. If your using a regular tubing cutter it will work harden the Stainless making it almost impossible to flare even with annealed tubing.
3. use a small chop saw like CHOP SAW LINKY to prevent # 2. NOTE: also available on AMZ
4. Make sure to lightly counter bore the tube after cutting
5. A high quality flaring tool is a must. Using the bar clamp type is a roll of the dice

MAKE SURE TO blow compressed air through the lines when done or before install.