Ensure that the devices are within range of each other (varies by device and device settings), that both are on/awake, and that neither device is in a location that is known to attenuate radio signals significantly (i.e. in a metal enclosure or surrounded by water).
Next check the channel configuration of each device. Network Key, RF Frequency, Channel ID, and Channel Period should match and the Channel Types should be complementary (i.e. one master and one slave). Also ensure that you configure the channel in the order defined the ANT Message Protocol and Usage document, so that you don't accidentally set any values back to their defaults (i.e. assign channel before configuring it).
If using exclusion/inclusion lists, make sure that they are not causing the slave to ignore the master.
If using a wild card search for pairing, make sure that the pairing bit configuration matches in both devices (i.e., if the pairing bit is set on the master, it should also be set on the slave).
ANTware II and SimulANT+ are useful for ensuring that devices are capable of transmitting/receiving messages. See Chapter 5 of the ANT Message Protocol and Usage document for a full explanation of ANT Channels.