What you have not told us is which Serial ports on the RFX 9600 extender you are using. A long, long time ago, I put up a post about Serial Ports 3 and 4 using a single UART.

The symptoms you are describing sound like exactly how the issue discussed below would appear to the user.

See the 2nd page of this thread.

[Link: remotecentral.com]

Result: You cannot do receives or sends on both at the same time. Note that when you send any data to the RFX for transmission, the receive buffer on the same port is cleared prior to the send. Any data that was there is gone. This can cause issues if you are expecting to do receive loops to capture an event stream coming from a piece of equipment.