Oh man, that did drive you crazy for a long time! Glad you did find the cause & corrected it!
I don't know how many times I've read the phrase "back probing the connector will allow...." and I'd look at the connector in question and look at my test light and think..."no, this is not a good idea Tom!"
Phone wire & a small needle nose or tweezers works nicely with out distorting the original connections.
Tom