The ferrule seals in two places, one is the tapered portion that goes into the L-fitting, the other is against the copper tube where it gets squeezed down when you tighten the nut. It has no choice but to compress, it has no where to go. If it's leaking along the tube, no amount of tape or paste is going to seal that. An axial scratch will leak if the copper tube or brass ferrule can't deform enough to fill the scratch.
If the ferrule isn't tight on the tubing when you remove it to check, there's a problem with either the ferrule or the tubing. Once you tighten it the first time, it's there for good, or should be.
I hand tighten the nut, then force the tube into the fitting as far as it will go, then tighten the nut while holding the tube in place so it can't spring back.
Standard compression fittings aren't great quality stuff these days, you may have a bad one. If you have enough slack left and don't want to use a SS flex hose, I'd recommend going to a shop that sells quality fittings, and ask for a Parker or Swage-Lok fitting to replace the compression fitting you have now. A hydraulic shop should have access to these.
They're meant to seal several thousand psi, domestic water pressure is no challenge to them. The two piece ferrule is better quality, and the price will reflect that. Ask the counter sales guy to take it apart in front of you and show you how it's supposed to be assembled, it's easy to mix up the pieces.
Sean
