Relentless Attacks should definitely be applying to the Captain as well. It's working that way for me.
Sounds like it's either a bug or you've got a display option toggled on that's hiding your buffs. Are you still seeing other buffs like Motivating Speech, Battle-Readied, etc? What level are you? (The buff should be available at any point after unlocking specialisations, but knowing what level you're at could help figure out what the source of the bug is, if there is one.)
Cainwen Ciaphas, Captain of Crickhollow, HERO OF MIDDLE EARTH!!!
I will not be pushed, filed, stamped, indexed, briefed, debriefed, or numbered! My opinions are my own!