I have discussed this issue with a number of people of varying professions,

My advice :

I have heard it said that blowing harder or softer can alter the pitch and make a note sound a bit flat (blow softer) or a bit sharp (blow harder).

Sometimes dirt can build up around the finger holes and in the mouthpiece. Maybe cleaning the out-of-tune whistle will help.

Psycologists advice :

Maybe once you start to believe a note is sharp or flat then there is more likelihood that you will actually hear it as being sharp or flat simply because of the way your mind and senses interact.

Psychoanalysts advice :

You need to ask yourself 'Why have I lost confidence in the sound of my whistle ?'.'Is it the whistle that needs tuned or is there a bum note somewhere in my life ?'

Spirtualists advice :

The six holes on your whistle represent the six seasons, spring, summer, autumn, winter, life, death. Your autumn is not aligned with your winter causing life to feud with death. Alas.

Dentists advice :

Your teeth need realigning.