The DM is seeing it as since the secondary says daily, it can be used once.
I understand that the stance is active until end of combat, stance change, or unconscious.
The Stance is a daily power.
But the Duelist Prowess attack power shows its little power bar color is also a daily and dailies can only be used once until an extended rest, correct?
So I would only be able to use the secondary power once even though the stance is still active or does it work a different way?
I found this page How often can secondary powers be used? that gives great examples, but it says that the specific wording in the parent power trumps the general rule of the secondary power being a daily.
With that said this is what the power says.
The way you stand appears to allow a foe an opening, but in truth your skill is such that you use your foe’s attack to draw it in for your own quick strike.
Daily, Martial, Stance, Minor Action, Personal
Effect: Until the stance ends, each time an enemy hits or misses you, you can use the Duelist's Prowess Attack power against it.
Duelist's Prowess Attack
Daily, Martial, Weapon, Immediate Interrupt, Melee weapon
Requirement: The power Duelist's Prowess must be active in order to use this power.
Requirement: You must be wielding a light blade.
Trigger: An enemy adjacent to you attacks you
Target: The triggering enemy
Attack: Dexterity vs. Reflex
Hit: 1[W] + Dexterity modifier damage.
Also if you can point me to any specific pages in the books or compendium that say the power specifics trump general ruling that would be great.