@bcdaus from what I understand, ! means failed while ? means unknown. Unknown is used when the radio hardware does not support ack at all (so there is no way to know whether the message was received by the next node) https://www.mysensors.org/apidocs-beta/group__MyTransportgrp.html has some documentation.

I am going to rework the system so it uses an Easy Pcb and a Pro Mini rather than the Leonardo board. I think I might be seeing a voltage drop in my point to point dupont wiring from the Arduino to the Radio. I'll update you all once I get a chance to make the change. Bill