Backups failing despite many settings that should prevent it

Hi all,

I've been getting some backup failures when trying to back up a fairly
large database. I've tried setting:

* vacuum_defer_cleanup_age very high on the master
* hot_standby_feedback = on on the slave
* max_standby_streaming_delay = 300s and max_standby_archive_delay = 300s
on the slave

Nothing seems to stop it failing intermittently. The error I'm getting is
below:
pg_dump: Dumping the contents of table "impressions" failed:
PQgetResult() failed.
pg_dump: Error message from server: ERROR: canceling statement due to
conflict with recovery
DETAIL: User was holding a relation lock for too long.
pg_dump: The command was: COPY public.impressions (id, ... ) TO stdout;
The following system errors were returned:
Errno::EPERM: Operation not permitted - 'pg_dump' returned exit code: 1

Any ideas what setting I'm missing?

Thanks,

John