pg_stat: last vacuum and analyze times are not being updated - v8.3.5

PostgreSQL Version: 8.3.5

OS Version: FreeBSD 7.0 i386

Issue: the last_vacuum column is not
being updated following a manual vacuum against a table or database.
This was highlighted recently when I configured the check_postgresql.pl
script to be used from NAGIOS.

Background:

Drilling down through the pg_stat_all_tables view, the underlying
functions

* pg_stat_get_last_vacuum_time(),

* pg_stat_get_last_autovacuum_time(),

* pg_stat_get_last_analyze_time() and

* pg_stat_get_last_autoanalyze_time()

are not returning anything even though I have run manual and auto
vacuums against these tables recently, along with ANALYZE.

Whether it makes a difference, I can confirm that track_counts and
track_activities are both on in postgresql.conf. The stats collection
service is also running on the server - I've verified this by looking at
the processes on the server.

This is causing issues with both our monitoring and also our confidence
that the autovacuum service is running as it should. We have a standby
server with identical configuration (as far as I can see) and this is
correctly keeping the stats up to date. The only difference is postgres
has been running well over a year on the server with the issue.

Many thanks in advance,

Steve Jones