I agree with Tony that the threads and the flare itself should have some sort of lubrication, personally I use some brake fluid. I have seen what appeared to be a perfect double flare give trouble and found that it was flared to wide. The outside edges of the flare caused it to not seat down into the fitting. When doing the second step of a double flare caution should be used to not push tool to far and make the flare bigger than it should be.
