Moisture accumulation in the thipple is a problem for me as well. Some players use a style that uses less tounging attacks and that can help a bit. In a session you can stop playing for a few seconds, move the whistle further into your mouth so that the sound hole is blocked and then blow into the whistle. This will clear out built up moisture. I do this in between tunes routinely.

I think other players only seem to do better with your whistle. When you are playing the sound travels away from you and is harder to hear in a loud session. Its easier to hear a whistle when someone else is playing, especially on low notes, during sessions.