There is one possibility that hasn't been mentioned.

If the faces of the bb shell are not parallel, the lock ring has a pretty good chance of loosening up.

"Facing" or milling the faces of the shell removes the paint and excess metal. It allows the lock ring a
smooth surface to rest against.

Tools are expensive for this, but it is truly the best way to get top performance out of
any bb or headset- especially precision versions.

rusty