AP sometimes cannot join WLC

Hi AP cannot join WLC. The AP is not in management vlan. I added command ip helper-address and ip forward usp 5246. It began to work. but I do not know why it cannot work now. The below is error message from the AP console. Thank you



Jul 10 17:30:15.161: %LINK-5-CHANGED: Interface Dot11Radio0, changed state to administratively down
*Jul 10 17:30:15.161: %LINK-5-CHANGED: Interface Dot11Radio1, changed state to administratively down
*Jul 10 17:30:15.171: %LINK-5-CHANGED: Interface Dot11Radio0, changed state to reset
*Jul 10 17:30:15.211:  status of voice_diag_test from WLC is false
*Jul 10 17:30:15.211: %LINK-3-UPDOWN: Interface Dot11Radio1, changed state to up
*Jul 10 17:30:15.231: %LINK-3-UPDOWN: Interface Dot11Radio0, changed state to up
*Jul 10 17:30:15.243: %LINK-5-CHANGED: Interface Dot11Radio1, changed state to reset
*Jul 10 17:31:26.000: %CAPWAP-5-DTLSREQSEND: DTLS connection request sent peer_ip: 10.0.0.200 peer_port: 5246
*Jul 10 17:31:26.000: %CAPWAP-5-CHANGED: CAPWAP changed state to  
*Jul 10 17:31:55.999: DTLS_CLIENT_ERROR: ../dtls/dtls_connection_db.c:2017 Max retransmission count reached!
*Jul 10 17:31:55.999: %DTLS-3-HANDSHAKE_RETRANSMIT: Max retransmit count for 10.0.0.200 is reached.
*Jul 10 17:32:26.048: %DTLS-5-SEND_ALERT: Send FATAL : Close notify Alert to 10.0.0.200:5246
*Jul 10 17:31:26.000: %CAPWAP-5-DTLSREQSEND: DTLS connection request sent peer_ip: 10.0.0.200 peer_port: 5246
*Jul 10 17:31:55.999: DTLS_CLIENT_ERROR: ../dtls/dtls_connection_db.c:2017 Max retransmission count reached!
*Jul 10 17:31:55.999: %DTLS-3-HANDSHAKE_RETRANSMIT: Max retransmit count for 10.0.0.200 is reached.
*Jul 10 17:32:26.048: %DTLS-5-SEND_ALERT: Send FATAL : Close notify Alert to 10.0.0.200:5246
*Jul 10 17:32:26.103: %CAPWAP-5-CHANGED: CAPWAP changed state to DISCOVERY
*Jul 10 17:32:26.103: %CAPWAP-5-CHANGED: CAPWAP changed state to DISCOVERY
*Jul 10 17:32:26.163: %LINK-5-CHANGED: Interface Dot11Radio0, changed state to administratively down
*Jul 10 17:32:26.163: %LINK-5-CHANGED: Interface Dot11Radio1, changed state to administratively down
*Jul 10 17:32:26.173: %LINK-5-CHANGED: Interface Dot11Radio0, changed state to reset
*Jul 10 17:32:26.194:  status of voice_diag_test from WLC is false
*Jul 10 17:32:26.194: %LINK-3-UPDOWN: Interface Dot11Radio1, changed state to up
*Jul 10 17:32:26.214: %LINK-3-UPDOWN: Interface Dot11Radio0, changed state to up