Lambert, who took over as Stoke boss five weeks ago, is baffled as finding the net is not a problem for the 24-year-old in training.
“I don't know why it's not happening for him," he said. “All I can see is what's happening in front of me.
“I tell you what — what a natural finisher. He's absolutely brilliant."
Lambert added: “The boy just needs a wee bit of confidence and game time. I thought he did well in our last game against Brighton when he came on. He just needs that goal again to get going."