We tested this hard and could not make it fail: a Dawn Warrior with Final Attack Lv11, the buff cast once and then 60 Power Strikes / Slash Blasts on a monster that does not die, with a sword. Final Attack fired 12 times out of 33 rolls (about the 22% you would expect), also when the monster is controlled by the server and when attacking as fast as possible. Things that DO stop it, matching the original game: it only follows Power Strike, Slash Blast and Brandish (never a normal attack), only with a one- or two-handed SWORD (not axes, blunt weapons or spears), only while the Final Attack buff is up, and only when the monster survives the hit that triggered it. If it still never fires for you, please tell us your exact weapon, the skill you used and whether the buff icon was showing, and we will look again.