autovacuum ignores some tables

hi,

postgresql8.4.7 here.

i checked the pg_stat_user_tables table, and it have a lot of rows
there where the "last_autovacuum" and/or "last_autoanalyze" are null.
does this mean that autovacuum never worked on those tables?

roughly 70% of all the tables have null in those fields..
in those never-autovacuumed tables there are tables that are quite
big, and also have a lot of activity, so it's not that they never
needed vacuuming...

i wonder why autovacuum ignored them. i checked my settings with "SHOW
ALL" in psql, and the corresponding settings are:

autovacuum on
autovacuum_analyze_scale_factor 0.1
autovacuum_analyze_threshold 50
autovacuum_freeze_max_age 200000000
autovacuum_max_workers 3
autovacuum_naptime 1min
autovacuum_vacuum_cost_delay 20ms
autovacuum_vacuum_cost_limit -1
autovacuum_vacuum_scale_factor 0.2
autovacuum_vacuum_threshold 50
track_counts on

any ideas why autovacuum ignores some of the tables?

thanks,
gabor