Warband Gear Upgrade Discount Not Working for Other Warband Characters

While participating in available endgame content in World of Warcraft such as World Quests, Delves, Dungeons, Raids, and PvP, you will earn two currencies, Stones and Crests. These have distinct names to separate them per active Season. For example, in The War Within's Season 1 they are called Valorstones and Harbinger Crests. In Season 2, we still use Valorstones but the crests are now called Undermine Crests.

These currencies can be used to upgrade eligible endgame gear to higher item levels, limited to their named upgrade track. Any gear you loot that is lower item level than your currently progressed item level for that gear slot will be eligible for an upgrade with a discount on Valorstones and/or Harbinger Crests. These discounted upgrades will go up to either the maximum item level for that item's upgrade track, or whatever your highest item level in that gear slot you've previously earned is.

The Stone discount is Warband-wide. That means the discounts will be decided by the character in your Warband with the highest item level in that gear slot.

The Crest discount is not Warband-wide until you've earned specific achievements for reaching item level milestones on the character in your Warband with the highest item level in all gear slots. For example, the Harbinger of the Weathered achievement requires that a character in your Warband has reached item level 593 in every armor slot. Once earned, characters across your Warband have a Harbinger Crest discount on upgrading their items up to item level 593.

Note that only items and gear you've looted directly, that are bound to your character, will contribute to this discount mechanic. Items you receive via trade from other players do not count.

Please check this Wowhead guide for more information.