Hi,
What is the firmware of these 8821's? If running the 11.0.3 firmware then please make sure that you have the latest device pack for cucm 10.5.2 installed as per the release notes of the firmware
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cuipph/8821/firmware/11-0-3/w881_b_cisco-8821-rns-110003.html
If running a lower versions try upgrading to the latest one and see if that helps, else it will require pcap's from the WLC and CUCM to understand which packets are dropping and in which direction that cause the phone not to register back.
HTH
Manish