My alt is a Level 30 Evie, just got Bind Arrow. I was able to use it fine in one mission, but then after I switched to Scythe and back, I can't use it even when my bar is charged! I'm certain I am pressing the correct keys...maybe it's because my Bind Arrow is Skill Level F? (although I'm not sure how that would matter)
Does it have something to do with changing weapons?