i'm currently on Seravi route on second playthrough and i have a full upgraded armor of seravi from my last play and i'm already at chapter 9 but that specific armor won't trigger it scene while others did just when i upgrade them. some other armor with almost full upgrade also has same problems.

Which armors do you have problems with?

Sprite tunic scene is in Yuidora Woodland. Harmonic tunic, in Amurent Forestland.
You should get the dialogue when you have upgraded the armor and unlocked the location.

In NG+, most of these dialogues appear simultaneously right after you get the heroine's first H-scene. Maybe you missed it? Try equipping the armors and checking the locations.