Terry Carr wrote:
Is there some secret that I'm missing to get this to work?
Optimised changing does not prevent a device from reaching 100% but monitors the charge so that is will occur when it can predict that it will be connected to the charger for an extended period - overnight for example.
See About Optimized Battery Charging on your Apple Watch - Apple Support
which says
...your watch delays charging past 80% in certain situations. In these situations, the watch's battery level remains between 75% and 80%. Your watch uses on-device machine learning to learn your daily charging routine so that Optimized Battery Charging activates only when your watch predicts it will be connected to a charger for an extended period of time.