It sounds like you don't have an Engine Brake but instead have a PacBrake Exhaust Brake - 2 different animals.
I too have the PacBrake (PB) and have inconsistent operation. When I start off in the morning the PB works perfectly but later on, maybe a couple hours, it doesn't work when I need it to but does work when I don't really need it, like to slow down the coach while on Cruise Control. I have to start downshifting and then I'll see the tranny show a 2 and I know that now it's working. I suspect that the Throttle Position Sensor (TPS) is flaking out, even though it is relatively new. My coach still has a cable going from the fuel pedal all the way back to the engine - yours may be electronic (I have Cummins).
What I did was cut the correct wire from the Transmission Control Module (TCM) and insert a SPDT switch so I can route the TCM signal or a ground thru a foot pedal to the Vehicle Interface Module (VIM) that controls the power to the PB. When the PB system isn't working correctly I flip the switch and step on the foot pedal while starting the downshifting process. The system usually kicks in during this process.
If you want to learn more let me know.
If you want to have your TCM (inside your tranny control on the dash) checked out contact
https://www.transmissioninstruments.com/. I sent mine in and they tested, found no problem and there was no charge except for shipping. Turn around was quick.