My Captain also does not always, or even usually, get the defeat response he is supposed to get from Elendil's Fury.
From the wiki, "Blocking, Parrying and Evading attacks build stacks of Elendil's Fury. At 5 stacks, you channel this Fury into your next Blade of Elendil for additional damage and grant yourself a defeat event."
It has been my anecdotal experience this is not working as intended. Hitting with (Improved) Blade of Elendil SHOULD be granting defeat responses much more often. If there is an internal cooldown or some other trigger, that should be made known to us, please.
Bruuwnor, Dwarf Champion of Arkenstone, Pipeweed and Ale