vacuum question

Hi all;

we have a large table that gets a lot of churn throughout the day.

performance has dropped off a cliff. A vacuum verbose on the table showed us
this:

INFO: "action_rollup_notifier": found 0 removable, 34391214 nonremovable row
versions in 152175 pages
DETAIL: 22424476 dead row versions cannot be removed yet.
There were 0 unused item pointers.
2 pages contain useful free space.
0 pages are entirely empty.
CPU 0.03s/2.58u sec elapsed 4.89 sec.
VACUUM

were running at isolation level 1 (autocommit), using serialized transactions
and a persistent db connection.

Anyone have any suggestions per why these rows cannot be removed yet? As far
as we can see these rows should be 'frozen' at this point. Can we force the
issue somehow?

Thanks in advance