Hi Paulette, It would be terrific if whistles were an easy thing when it comes to tuning. However, there are quite a few things affecting the pitch.
Firstly, the breath pressure. If you play a single note into a tuner, you will find that most whistles will go sharp as you blow harder. I think the Sweet-spot that Ronaldo has mentioned (correct me if I’m wrong) is where the note is most resonant and also most comfortable in context with the surrounding notes on the whistle - it is dependant on the breath pressure you apply over the whole scale and can be a bit subjective.
Secondly, the temperature. Higher temperature = sharper note. The ambient temperature will balance out with the body-temperature of your breath. The different parts of your whistle will respond to temperature in different ways and the whole instrument will come to a balance at some point - this is why a whislte needs to be “warmed-up”, some more than others.
Thirdly, the humidity. Higher humidity = sharper note. Similar to temperature, the humidity in the tube will gain a balance between your breath and the ambient humidity.
Fourthly, Altitude. Higher altitude = sharper note.
Fifthly, scale temper. The pitch change between notes can vary depending on how the whistle was made. Most makers can tune a whistle to achieve a certain “temperament” depending on what the maker prefers or how the player wants it to be. For instance, it may be best to have an “even tempered” scale if you are playing with guitars, pianos or other even-tempered instruments. A “just temper” might be the choice if you are playing with fiddles or simply prefer it. Other hybrid temperaments can be used with trade-offs between tuning, what keys you want to play and style. Or the whistle might just be badly tuned.
Sixthly, the bore architecture of the whistle has a large affect on how the tuning behaves in the second octave.
There are other factors as well.
All these things combine to make a whistle very “organic” in the way it plays - this is part of the character of the instrument, part of the “mystique” and allows for that great expressive dymanic. Depending on the whistle, it can be more or less easy to remain in tune across the range of notes you have.
Also keep in mind that A=440 is only a reference - in some keys A is not = 440 hertz. Performers and sound engineers will have their various reasons for diverging from the reference.
It is a good idea to have a tunable whistle - this does not mean you need to spend a fortune - even the cheapest of whistles can be made tunable with a simple “tweek”. ( http://www.chiffandfipple.com/tweak.html . Steps 1 to 6).
Learning to tame all this and get music out of the little beasty is part of the game
It does get better!