I don't think flair nut and compression fittings are the same thing. A compression fitting involves a gland (or sleeve) you slide over the tube after the end is cut off, a flair nut type involves flairing out the end and there is a male mating face on the other side. It could be leaking for a couple of reasons.
It could be not tight enough or possibly there is some crap on the face that isn't letting it get a good seal. Or the treads are cocked and it's not sealing properly. Have you tried taking it apart and looking at the face of each side? I think brake line flairs are double backed flairs, not something you can do w/ a normal flair nut tool. You might have to get a whole new line rather than have it cut back and re-flaired