How to tell when a plumbing fitting is tightened correctly?
Compression fittings do not use tape, so that's one mistake to fix. Tape seals a leak in threads, but a compression fitting is sealed at the ferrule, not at the threads. Leaks are frequently caused by over tightening, so this is certainly a possibility in your situation. Here are my suggestions:


Replace the nut and ferrule. The ferrule gets deformed when compressed, so a new ferrule won't have the deformations from the previous valve.
Hand tighten the assembly, though I typically use a wrench and guess where I could likely get it by hand without destroying my fingers. Add another half turn by wrench.
Test for leaks, and if found, add another quarter to half a turn until the leak stops. By doing this in iterations, you minimize the risk of over tightening. You can always tighten it more if you under tightened it, but once you over tighten, you have to get a new ferrule.