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,