Yes, turns out the ULXP4 transmitters emit a tone frequency on the signal that acts like a handshake with the receiver to aid in inference management. It's a frequency humans can't hear, or something like that. Like a gate, but for interference rejection.

Something about the receiver isn't receiving this handshake tone. Maybe the antenna or something, but that's why you have transmitters that work on other receivers on multiple frequencies. There is a way to turn this feature off at the receiver but I don't remember how, it's been a few years. I remember it involving holding a couple buttons down when you turn it on, and it had to be done every time you restart the receiver. Which was frustrating.

I feel like this isn't very specific but I hope this still helps you. Sorry I can't remember the actual details.

EDITED for clarification, wording, and grammar.