Update: It was a bug… Just cutted the same gem at the same level and now works okay. Thanks all!

Try changing it to weapon set 2 and then back too 1

Open your skill tab, choose the arrow cursor on the right of the Herald to view the detail tab. In the bottom right of that detail tab has 2 check boxes to assign to 2 weapon sets, those 2 are bugged. Keep checking and uncheck them until the Herald turn on.

Do you still meet the attribute requirements? You might have been using different gear that gave different attribute bonuses.

If you do, then it could be a bug, you might need to cut a new Herald of Thunder gem for it to work again. EDIT: before cutting a new version of the gem try messing with the checkboxes for weapon set I and II at the bottom of the detail screen you see.

Looks like one of your weapon sets isn’t able to activate it. Most likely either because you need more spirit or higher stats on that weapon set. Does your quarterstaff have a lot of dex on it possibly? Could be that with that equipped you meet the dex requirements but not with set 2.