Never had a problem with this until XCode 8 and Sierra. Method signature = "register:messageNamed:withBlock:". Nothing was flagged as an error, but the word “register” is red color in XCode 8. DoubleClick will not link to it. The routine does not show in the menu methods list. No messages (MPC) were getting through.
Changing the method to "registerMsg:messageNamed:withBlock:" fixed it. This is very disconcerting because I never thought part of a message signature could cause something like this, and it permeates six apps we have in the app store. Lots of work and updates ahead to cure this.
Not sure if this should be considered a bug.