New electro couplers work on test track, not on layout. What gives?

I'm guessing that there is a track voltage drop at the time the coupler fires on the layout.  What you are describing is a coupler that isn't receiving enough voltage to actually open.  I suspect voltage drops where the actual locomotive is, and it's probably getting less than the indicated voltage on the ZW-L.

John, my layout is 16' x 7'.  I have two loops and a yard. I just checked the voltage. At the ZWL terminal it's 18.3...on the track, depending on what is on it and turned on, it drops as much as 1-2v.  I tried a few MTH locos, and all the couplers fired. I tried two other Lionel locos and they fired on one, not the other.  I also noticed a small voltage drop as you said when the couplers are fired.  Next I moved the loco to my second, independently powered, loop (Throttle lever A). The new couplers on the GP35 fired consistently no matter where on the loop. Voltage was about the same, within 2.-.4.  Then I moved it back to the outer loop and the couplers opened consistently in certain places.  So, I guess as you noted, it's the voltage. I must be operating right at the margin for some of these locos couplers and any small drop means not enough oomph to open them.  What' s odd is they are now opening on the same area of the outer loop which they didn't before I sent my SOS.  What I need to explore is whether I can boost the output from the ZWL on the outer loop to say, 19V at the terminal and see if that fixes the problem. I'll clean the track thoroughly and I'm done.  Thanks for your help.  Any other suggestions welcome for sure.

I've noticed that some couplers are more voltage sensitive than others.  You might also try some dry lubricant to see if that helps.

I'd also look into the voltage drops.  I see around 18.8 volts all around my layout.  If I measure at the yard tracks and then start up a diesel with smoke and six passenger cars, I see a .2-.3 volt drop for the roughly 1 amp draw.  If I were seeing a couple of volts of drop for one train, I'd be concerned.

Oh, my. I’ll focus on the voltage drops. Thanks. It’s always, something. 😬