Xayah AA with feather passive doesn't kill hp on GP Barrels

When I played Xayah the enemy had Gangplank in toplane. When he would barrel and I had used for example my q and then went to auto attack a gp barrel, the hp didnt go down or if there was one attack needed I couldn't kill it. This made me unable to kill them in teamfights and give him the opportunity to just blast away

This happened the entirety of the game. Not just once, every single time.