8.1: autovacuum not doing its job?

hi list

 

on a 8.1 database, pgAdmin III warns me on several tables:

 

Running VACUUM recommended
The estimated rowcount on the table "forum.notifications" deviates significantly from the actual rowcount. You should run VACUUM ANALYZE on this table.

 

shouldn't autovacuum take care of this on a regular basis? how do i need to tweak the autovacuum settings to have proper vacuumed tables?

 

thanks for helping,

thomas