check_postgres reports most of my tables are unanalyzed for 8 weeks; why isn't autovacuum analyzing them?

Hi.  I've just discovered the check_postgres utility and am running
all the tests against my database.

The "last_analyze" test comes out critical - many tables unanalyzed
for 8 weeks.

I am running PostgreSQL 8.4.4 with default autovacuum settings.  I
thought autovacuum was
supposed to take care of running vacuum and analyze.

Per "last_analyze",  9 out of my 100 tables have been analyzed within
the last 24 hours.

Why aren't all my tables being analyzed?  How do I troubleshoot this, please?

Best,
-at

P.S. I checked the PostgreSQL logs, and in the last week, I just have
three messages with "vacuum" in them:
(I am running at default log levels)

2010-07-20 02:05:05 PDT ERROR:  canceling autovacuum task
2010-07-20 02:05:05 PDT CONTEXT:  automatic vacuum of table
"mydb.pg_catalog.pg_listener"

2010-07-20 07:27:14 PDT ERROR:  canceling autovacuum task
2010-07-20 07:27:14 PDT CONTEXT:  automatic vacuum of table
"mydb._slony_cluster.sl_event"

2010-07-20 07:27:53 PDT ERROR:  canceling autovacuum task
2010-07-20 07:27:53 PDT CONTEXT:  automatic vacuum of table
"mydb._slony_cluster.sl_log_2"