I have tried everything from re synchronising databases, repairing databases, cleaning the sync store,deleting the app and reinstalling. It seems to be just DTTG. It syncs ok between my iMac and Macbook. It is uses Cloudkit.
I may be entering the sunk cost fallacy zone here. I think I am going to stop using it. The reality is I very rarely would need it on my iPhone, so it is not a productive use of time to be fiddling about with it trying to get it to work.
Just use something else. CloudKit is broken.
More diplomatically: CloudKit is well known to be the least reliable syncing method. You can simply use Bonjour instead. It is supposedly faster and in any case much more reliable.