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