Well, it seems that a slight deficit in track voltage WILL affect coupler activation. I found a few spots that were in the mid 16 v range so today I spent time running jumper wires. Now, I have a solid 17.9 - 18.0 everywhere.
So given that, NOW every coupler on every loco works flawlessly.
Apparently, my two recent trackside additions lowered the track voltage enough to cause the problems.