You must have the transponder wired in parallel to all SkyView screens or you will get a red-X on the non-connected screens. This is the first thing to check.
You also need to have the transponder configured on the correct serial port on each of the screens it's connected to. It could be connected but you could have the wrong serial port configured in setup on the screen with the red-X.
The final possibility is that you have a broken wire coming from the transponder TX to the RX on the non-working screen.
Please also be aware that until this issue is solved, your transponder may not function at times, as the system will sometimes switch control between screens and will not have a connection when using the non-functional screen.
