You may not have enough line when doing the first flare or the line is slipping in the clamp part of the tool.
To do a double flare, put the line in the clamp piece and extend it upward, the same height as the shoulder of the double flaring insert( sit the insert upside down on the clamp to judge how much you want to sit up out of the clamp), then lock it in place, set the insert in the tube/line and compress until it bottoms out on the clamp. Remove the insert double flaring piece and retighten the the tool down to roll the flare inward. A good flare tool is needed for this.
Practice on a spare piece of line, it's easy once you know how much material you need to do this.
Hope that helps.