problem on replication EFM (WAL LSN not match)

I have a postgres cluster (edb) , when i check the status i have this output :

Cluster Status: efm

Agent Type  Address              Agent  DB       VIP
-----------------------------------------------------------------------
Master      xxxxxxxxxxxx.com UP     UP       xxxxxVIP.net*
Standby     xxxxxxxxxxxx.com UP     UP       xxxxxVIP.net

Allowed node host list:
xxxxxxxxxxxxxx.com xx.xx.xx.xx

Membership coordinator: xxxxxxxxxxxxxxxx.com

Standby priority host list:
xxxxxxxxxxxxxxx.com

Promote Status:

DB Type     Address              WAL Received LSN   WAL Replayed LSN   Info
---------------------------------------------------------------------------
Master      xxxxxxxxxxxxxxxxx.com                    71A/69CD6000
Standby     xxxxxxxxxxxxxxxxx.com 71A/69CD6000       71A/69CF5A80

One or more standby databases are not in sync with the master database. The following node(s) returned a WAL LSN that does not match the master: xxxxxxxxxxxxxxxxx.com

Note the standby is not in sync withe master. Can you help me understand why, and what I can do to fix this?