Finally I have found something that works for me, Galaxy S9. While a very long way from home, I disabled the preset Home setting in Smart Lock and deleted my own additions. I set another home address by finding my house on the map and setting that. It works now... consistently. Maybe the Google GPS settings were not precise enough to set in situ?
When I returned home my Smart Lock worked well. Maybe worth a try for someone else who is well away from home.