Over on the Atlas Rescue Forum, several people wrote about a (Sergent) coupler problem. I quote one of them:
"I can close it with a magnet but it should close on its own without using a magnet looks like the metal ball bearing i blocking it, the knuckle should move that ball up into the upper part of the coupler but its jamming and wont budge."
Several of us either said or hinted that they screwed up assembly.
A couple of days ago, I put a coupler together and experienced the same problem. I had read the topic previously, so I was very careful about what and where I filed metal. I DID NOT remove any metal that changed the working surfaces except to gently smooth them.
Anyway, I've just spent some time examining this coupler. What's happening is the ball is falling from its raised position, partially or completely, behind the knuckle piece. When this happens, the knuckle cannot be closed. If one then puts the magnet above the coupler, the knuckle CAN be closed. This is exactly what the other guys described.
It would appear that the knuckle may be opening a bit too far, thus allowing the ball to drop down. I did not remove ANY metal in the locations that would control this.
I was about to report my experience back to the Atlas Rescue Forum, but I thought I would stop here first, in case someone can supply additional (helpful) information.