logical standby apply hang

Experts,in alert:

LSP0: warning -- apply server 22, sid 4697 waiting for event (since 310 seconds):
Sun Nov 03 06:54:18 2013
LSP0: warning -- apply server 16, sid 2819 waiting for event (since 891 seconds):
Sun Nov 03 06:56:35 2013
LSP0: warning -- apply server 12, sid 1880 waiting for event (since 560 seconds):

 cat a.sql
column status format a50 wrap
column type format a11
set numwidth 15

SELECT TYPE, STATUS, HIGH_SCN   FROM V$LOGSTDBY;

set numwidth 15
col NEWEST_SCN format 9999999999999999;
col APPLIED_SCN format 9999999999999999;
col READ_SCN format 9999999999999999;
set lines 200;
select
      (case
      when newest_scn = applied_scn then 'Done'
      when newest_scn <= applied_scn + 9 then 'Done?'
      when newest_scn > (select max(next_change#) from dba_logstdby_log)