table "db1.public.a" and all behind "db1.public.a" tables did not do analyzing

when I used vacuumdb -d db1 -z -v > /BACKUP/script/db1.$LOG_DATE.log 2>&1
it occued this error [automatic vacuum of table "db1.public.a": could not
(re)acquire exclusive lock for truncat]
and table "db1.public.a" and all behind "db1.public.a" tables did not do
analyzing.
I want to know:
1.why table "db1.public.a" and all behind "db1.public.a" tables did not do
analyzing?

log is below:
#PostGreSQL: VACUUM 2014-02-07 05:36:16 CST user db1 [local] CSTINFO:
vacuuming "public.t1"
#PostGreSQL: VACUUM 2014-02-07 05:36:16 CST user db1 [local] CSTINFO:
analyzing "public.t1"
#PostGreSQL: VACUUM 2014-02-07 05:36:16 CST user db1 [local] CSTINFO:
vacuuming "public.t2"
#PostGreSQL: VACUUM 2014-02-07 05:36:16 CST user db1 [local] CSTINFO:
analyzing "public.t2"

#PostGreSQL: VACUUM 2014-02-07 05:36:16 CST user db1 [local] CSTINFO:
vacuuming "public.a"
#PostGreSQL: VACUUM 2014-02-07 05:36:18 CST user db1 [local] CSTLOG:
automatic vacuum of table "db1.public.a": could not (re)acquire exclusive
lock for truncate scan

#PostGreSQL: VACUUM 2014-02-07 05:36:18 CST user db1 [local] CSTINFO:
vacuuming "public.b"
#PostGreSQL: VACUUM 2014-02-07 05:36:18 CST user db1 [local] CSTINFO:
vacuuming "public.c"
#PostGreSQL: VACUUM 2014-02-07 05:36:20 CST user db1 [local] CSTINFO:
vacuuming "public.d"
#PostGreSQL: VACUUM 2014-02-07 05:36:20 CST user db1 [local] CSTINFO:
vacuuming "public.e"
#PostGreSQL: VACUUM 2014-02-07 05:36:27 CST user db1 [local] CSTINFO:
vacuuming "public.f"
#PostGreSQL: VACUUM 2014-02-07 05:36:27 CST user db1 [local] CSTINFO:
vacuuming "public.g"
#PostGreSQL: VACUUM 2014-02-07 05:36:27 CST user db1 [local] CSTINFO:
vacuuming "public.h"
#PostGreSQL: VACUUM 2014-02-07 05:36:27 CST user db1 [local] CSTINFO:
vacuuming "public.i"
#PostGreSQL: VACUUM 2014-02-07 05:36:36 CST user db1 [local] CSTINFO:
vacuuming "public.j"
#PostGreSQL: VACUUM 2014-02-07 05:36:37 CST user db1 [local] CSTINFO:
vacuuming "public.k"
#PostGreSQL: VACUUM 2014-02-07 05:36:37 CST user db1 [local] CSTINFO:
vacuuming "public.m"

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