Back when the discussion started about the coupler manufacturing problems I looked through my couplers to see if I was affected. It turns out that while I had a few bad actors among the hundreds(!) of couplers I had assembled, none of my problems could be attributed to the design/manufacturing flaws that were brought to light.
After playing around with some of these couplers it finally occurred to me what was causing the problems I have been having of the coupler locking in the open position. On my problem couplers, when the knuckle is opened the ball is able to fall behind the knuckle tab and get trapped behind/slightly under the tab, which locks the coupler open and prevents the ball from being raised by the magnet or by gravity if the coupler is turned upside down. If I force the knuckle open just a bit more, the ball can return to the raised position and the knuckle closes freely again.
When I assembled these couplers (the plain Es purchased in packs of 144 and the lower shelf and double shelf Es in packs of 6), I noticed some - very few, in fact - of the knuckle castings had a little extra material on the end of the tab away from the knuckle itself. This extra material had to be removed and apparently I wasn't as careful as I could have been in removing it. I inadvertently made the knuckle tab a bit too short, which has the effect of allowing the knuckle a greater opening range of motion. This extra swing allows the ball to get trapped behind as I described above.
The simple solution to this problem is to replace all the offending knuckles. Frankly that's a bit of a pain, especially on those cars where the installation was a little difficult (I'm talking about you, Intermountain covered hopper kits) or where I've weathered everything and don't relish the thought of damaging the finish of the car. For those cars I have devised a solution that involves me cutting a small slice of 0.030" styrene square rod and glue it into the slot that the knuckle tab moves through at a point farthest away from the knuckle. This limits the opening range of the knuckle by a small amount, but by just enough to prevent the ball from falling behind the tab.
Anyway, I'm glad to have finally found not only the cause of this problem but also a solution.
Fort Worth, TX