The seal takes place between the inverted tubing flare and the corresponding "male" flare in the fitting.

If you have done the "loosen / tighten" routine more than 3 or more times, 1. Either the flare on the tube is bad,
2. The nut is bad,
3. The "seat" in the fitting is bad,
4. Or there is debris preventing the seal.

Disassemble, inspect the flare on the tube and seat in the fitting. Your problem should become apparent.