We have heard of customers in the past using an XBee module and SparkFun XBee Explorer with a 12-channel Maestro successfully (here is one example of another forum member).
From the information you have provided, I do not see anything obvious that would cause a problem. What is the behavior of the indicator LEDs on the Maestro (specifically the yellow one)? You can try to compare that behavior to the description in the “Indicator LEDs” section of the Maestro user’s guide to try to decipher what is happening.
-Brandon