Herald of Blood is not working.

It was working normally, but it stopped working after I slotted it into my weapon. Other heralds work, but this one isn't. I've checked absolutely everything and I don't know what the problem could be. It's a martial weapon, I have 30 spirit, it was working and now it's not anymore. When I equip it, it says "not usable with the current weapon set," even though it's the same set as when it was working.