I have two radios that have worked fine in the past when used as a buddy box.
The primary transmitting radio is a DX6i DSM2 and the slave box which my son uses is a JR 6ch standard TX with a digital display. Not sure of the model since I"m at work. The primary radio, DX6i flies the play fine. However, the buddy box radio, when used as a buddy box, has no up elevator control. It only has down. The LCD display on the slave shows the servo movement with up and down, but the servo doesn't go up. Since the plane flies fine on the primary box, obviously the servo works as does the primary radio. If you buddy them up, and give control to the slave radio, there is no up. Take control back to the primary and up works. I cannot test the slave radio directly on the plane since the plane receiver is DSM and the slave radio is not.
There are no mixes, all throws are good, no servo reversing, etc. Checked many times.
Seems the slave radio reads the servo moving on the LCD display, but no up in real life. What is the issue? Bad radio? Dirty contacts in the sticks? If dirty, then why does the display show the servo moving?
Thanks for any advice.
I might have to fine a standard receiver to test the slave transmitter.

That sounds like a weird one, especially down with no up.
I'd say take the box apart and clean the contacts.
Maybe one half is broken.
Check the contact plugs on the buddy cable as well.

can u just bind the JR 6ch directly to the plane? i assume both are DSM system.

My buddy box are dx7 + dx5. no issue at all after almost 2 years of abuse. next step will be dirty contact on radio sticks or buddy box cable plug hold.

good luck

Unfotunately only the master is DSM, the slave tx is standard frequencies. That would have made it simple for sure.

Take the back off the JR radio and look at the pot that is connected to the elevator stick . The pot has three wires connected to it. I would be willing to guess that one of the outer wires has come off as this would explain the problem you are having.

Well, in an attempt to remove the pot, I broke it completely. Time for a new radio unless I can find a new left stick.
All I really need is a 4ch trainer box really. I hate to throw out a good DX6i for sure.

Well you could get a TSIX by Orange. The trainer cord plug is different but it is a very good copy of the DX6i. Cheap too!