If a particular fitting doesnt seem to stop leaking, look at the matching cone in the fitting....where the tube seals to. IF there is a visible indentation on the cone, it is tough to make a good seal. This indentation means that the fitting was overtightened at some time or is plain worn. Brake line connectors work on the principle of mating cones EXACTLY like engine valves. The two mating surfaces have slightly different angles so there is a line of contact (a circle) that makes the seal. AN fittings are another great example of line contact.....also 60,000 psi water lines are sealed the same way.

Putting tape on the fitting threads does nothing; the seal is not on the threads nor around the tube.....it is at the mating cone.