XBee radios not communicating

I've been working on this for hours, including searching this and other forums and I don't have a clue what my problem here is.

I have two XBee S2C modules, and an XBee shield from iteadstudio (v1.1). I have those plugged into an Uno and a Mega. The radios are configured via XCTU and find each other in the network (while plugged into the Arduinos). I've uploaded simple programs - one with the Physical Pixel sketch and one alternately writing 'H' and 'L' to serial (as per this guide: https://www.arduino.cc/en/Guide/ArduinoWirelessShieldS2). On the transmitter, I can see that the D0 LED is briefly lighting up (and verified with some test code), but the receive does not respond.

Both are on the same channel, PAN, and one is setup as a router, and one as coordinator. Both are in API mode. Can anyone point me in the right direction?