67mm? You may have cross-threaded it. Are you possible it's not still able to be worked again if you do it slowly and carefully? Sometimes you can fix cross-threading "dents".
I don't think I cross threaded it. The lens screws onto the threaded caddy just fine.
I need to take another look at it with a magnifying glass. And yes, it is getting harder to see small things close up. I just don't want to force it and permanently fark it up.