ERA-DCKIT-2 
When troubleshooting problems with programming a transmitter to the LRA-DCRXS receiver, the red LED that appears in the strobe lens area is a key to programming. As mentioned in other FAQ questions, it's important to know that each zone on an LRA receiver can only have one transmitter/sensor programmed to that zone. If a zone is programmed to a transmitter/sensor, you cannot program another one unless you remove or erase the previously paired transmitter.
The LRA-DCRXS has two zones. Oftentimes, programming problems occur when a zone is already occupied and you are trying to program a transmitter to a zone that is already occupied. As mentioned above, the status of the red LED (once the unit is entered into program mode) is a signal to the zone programming.
If the red LED is blinking rapidly, then zone one is open. If the red LED is blinking slowly, that means zone one is occupied is zone two is open. If the red LED is not blinking (solid), that means both zones are already programmed to a transmitter.
If the transmitter/sensor will not pair to the receiver, the easiest thing to do is to delete the memory and reprogram the transmitter to the receiver. To remove the memory, consult the FAQ document, Deleting Memory from the LRA-DCRX & LRA-DCRXS Receiver. Once you have deleted the memory, re-program the transmitter/sensor to the receiver.
Another issue could be the compatibility between older models of the LRA products to newer models. For this, please consult the FAQ document, LRA Series Compatibility Notice.