problem connecting Zigbee Z-Stack EP to coordinator network

Hello,

I'm using a device with  TI's CC2520 and the standard Z-Stack (version 2.4.1) configured as an endpoint and I'm trying to connect to a network created by a XBee device from Digi.
I'm using a sniffer to see the messages exchanged.

The EP device is not connecting to the coordinator. 

Using the sniffer, I can see that the EP sends the Beacon Request to join the network, but there is no response from the coordinator, so the association procedure is not initiated.
The devices are on the same frequency channel and are both configured to use Zigbee Pro Profile.


Here is the beacon request:

Frame 684 (Length = 10 octets)
Sequence Number: 684
Channel Sequence Number: 684
Channel: 20
Time Stamp: Tue, 04 Dec 12, 17:11:13.997
Time Delta: +00:00:01.159
Frame Length: 10 octets
Capture Length: 10 octets
Link Quality Indication: 252
Receive Power: -25 dBm
FCS: Correct (0x01)
Protocol: IEEE 802.15.4
Packet Type: Command: Beacon Request
IEEE 802.15.4
Frame Control: 0x0803
.... .... .... .011 = Frame Type: Command (0x03)
.... .... .... 0... = Security Enabled: Disabled (0x00)
.... .... ...0 .... = Frame Pending: No data pending (0x00)
.... .... ..0. .... = Acknowledgment Request: No ACK required (0x00)
.... .... .0.. .... = PAN ID Compression: No PAN ID compression (0x00)
.... ..00 0... .... = Reserved: 0x00
.... 10.. .... .... = Destination Addressing Mode: Address field contains a 16-bit short address. (0x02)
..00 .... .... .... = Frame Version: IEEE Std 802.15.4-2003 (0x00)
00.. .... .... .... = Source Addressing Mode: PAN identifier and address fields are not present. (0x00)
Sequence Number: 199
Destination PAN Identifier: Broadcast (0xffff)
Destination Address: Broadcast (0xffff)
Command Frame Identifier: Beacon Request (0x07)
FCS: 0xffff
Beacon Request


I've also tried other devices to join the coordinator network (Digi router, Anaren EP, other TI EP's...) and they all connect. 
I don't understand why the coordinator doesn't reply to my device's beacon request. Do you have any suggestions on what could be happening?

Pedro