compression nut leaking on cold water to sink

I have a shutoff valve attached to the cold water (copper) line running up to vanity sink. Its connected via compression nut. I'm wondering if i've compressed too far? or not far enough? I have a steady leak of water droplets out of there, i'd say 5 drops per min? do i need to keep tightening or have i "ruined" the connection and need to re-cut the copper and reinstall a new compression valve?

Compression fittings.

It should have sealed as long as you either installed it out of the box, or sprayed some WD40 on the threads.

If you used Teflon tape or paste, you will want to remove that.