I just bought the Heroics option for other armor looks but neither the standard Hero's Forge Plate Helm Ornament or the Wanderer's Ayr'Dal Scalemail Helm Ornament will fit into my Helm of the Fallen Saint, all the other slots seem to work fine?
Armor augments often have appearance restrictions.. for example you can only use robe style looks on cloth and leather armor (appearance.. for example the Phase Spider Carapace looks like plate even though leather and chain classes can wear it. You can only use plate and chain type appearance augs in it, not leather or cloth.) I'd guess that's your issue.
Not all helms have a graphic. Verify that it actually shows a helm on your character. Likely it’s a graphics-less helm, thus the ornament will not work even if it has a slot for an ornament. Elemental helms for warriors have the same issue.
Get used to wearing non BIS helms or cultural if you want a plate/chain helm ornament to show up starting in PoP.
Not really true, graphicless helms just count as cloth type. I know because I used one without a graphic for years and could slip in the green wizard hat ornament when I wanted to play at being a ranger. Again I'm betting its due to the ones he is trying to use being either chain or plate appearance
I believe a bunch of the Vel-Pop era helms have a cloth graphic so cloth/leather ornaments should work. Opps, Yeah, what Zan said.
even if it has no looks it should be able to fit a plate graphics since it is obviously a plate armor set (and that goes for all of them) it is a PAID feature
