8.1.8 autovacuum missing databases

I have a Postgres 8.1.8 system with three databases.  Although
autovacuum is enabled (I'm using all default autovacuum configuration
settings) and pg_autovacuum is empty, the logs indicate that only
template1 and one of the three databases are being processed by
autovacuum.  Are there other reasons why the other two databases might
be skipped, or some way I could diagnose the issue?

Thanks,

    --Ian