Inside the spout, there are two upside-down V shape slots (these are the delicate two-tone reeds) and a tiny metal ball that sits over a hole. The ball should cover the opening when it's on the stove, pushing the steam upward through the reeds. When you tip the kettle to pour, the ball will move out of the way and allow for water flow.
Try to look through the screen to see if the ball is not sitting correctly, or if the reeds obstructed in any way. Blockage tends to be the most common issue. We also recommend verifying that water can still be poured out and the lid fits snug enough so no air escapes.
If you still experience inconsistent or no whistling at all, please reach out to us directly at hello@fellowproducts.com