I like the detergent idea, and plan to try it. Thanks.

Other thoughts: tuck the whistle into your waistband well before playing it, to warm it up to body temperature. Stay in the warmest part of the room. Since a metal whistle will cool fast, have a spare in place. They're cheap.

I bet that the other player you mention, who gets more volume from the same kind of instrument, just has a superior instrument. It may be the same maker, but more expensive. It may be broken in better. It may be random variation in the quality of the product. Maybe it's a "Wednesday whistle."

For best results in the long run, forget the low-cost whistle and play something of higher quality. Whistles are cheap for a reason.