One, you could replace with a good sounding whistle unit from a gifting tender. Two, as with many items made of same metal, there could be some warpage, even if minor. The slightest amount of warp will be enough to disrupt the necessary airflow. Check trueness of the whistle casing with a metal straightedge and use tape, etc. to make up for any lose of contact to gasket.
Jesse TCA 12-68275
