Nimbostratus
NimbostratusI raised a support case with F5 who said that this is due to a bug in the version my F5's were on (11.5.1). the cure is to upgrade to 11.6.0 which I've now done and which has solved the problem.
In the interim (it took a few weeks before I could update for various unrelated reasons), running this command at a bash prompt on the primary forces them back in sync
/usr/bin/tmsh modify cm device-group cluster name devices modify {primary {set-sync-leader } }
As we had a service that is stopped by the administrators daily, I created a cron job to run this command once a day in the early hours of the morning.
Martin S