Problem using PPM encoder with Pixhawk 4

I am using the Spektrum AR8020T receiver on several hexacopters. In order to connect it to the Pixhawk you need a PPM decoder between the receiver and the PPM slot on the Pixhawk. I have one setup that is working, but three others don’t. The ones that don’t work won’t let me calibrate the radio in QGC. I have tried several approaches but cannot find the problem. I exchanged the PPM encoders between drones, and the good one always works. Thinking I had 3 bad units, I returned them and got three new ones. But they don’t work either. I’m baffled by this.

Has anyone encountered this issue?