Vacuum issue

I have a database with a table that adds 30000 records a day, I am
delete records older than 2 days.
I am vacuuming it once a day , I am having an issue that the disk usage
is continually rising. i.e. the space been flagged as free by the vacuum
process isn't being reused.

Please find below a copy of the cron command being used.
0 0 * * * /data/newsam/scripts/samdatabasemaint.sh >/dev/null 2>&1

Relevent section from script

COMMAND="$prefix/bin/vacuumdb"

# What to use to shut down the postmaster

set -e

# Only start if we can find the postmaster.
test -x $DAEMON || exit 0

# Parse command line parameters.
su - $PGUSER -c "$COMMAND --analyze -v " >>$PGLOG 2>&1

exit 0

I orgionaly had autovacuum active but that encountered the sam issue

I am running Postgre 8.4 on a Susse 10.1

Thanks in advance for any help

E.J. Moloney