On the Spektrum radios, if you ever change recievers, you have to re-"bind" the Tx and the Rx.

In this case, you would put the BIND plug in the BAT slot of the Rx and power it on BEFORE the Tx, then turn on the Tx, the Tx goes through it's usual beeps and once again syncs with the new Rx.

Binding is the manner with which your device associates with the radio, and for reference - there is a page in the CX2 manual dedicated to the BIND plug, it's small though, I'm sure you flipped right past it. It's on page 40 if you ever need detailed instructions =D.

BZ