Knowing that tool, the problem must be tube preparation, because every one I know that has one do not have problems. If you are using a tube cutter to cut it, stop. A tubing cutter hardens the end of the tube as it cuts it, and puts a giant burr on the inside of the tubing. I cut it with a 3" cutoff wheel, and deburr inside and out. I get a perfect flare just about every time, maybe one in a hundred is bad.
One other thing, if you have an inch designated die in your flare tool, and are using it on metric tubing, or vise versa, it won't work right.