Although it is relatively uncommon, whistles can develop issues which affect the tone and playability.  Below are a few things to check before contacting us.
Do Bear in mind, if you are new to playing, some noises, squeaks and harmonics you may be hear when you play could be technique related.  Over-blowing and under-blowing will produce some noises which may seem strange or unexplainable initially.  The longer you have been playing the less likely it is to be this, and the more you will recognise which noises might be technique affected, or even what can be incorprated into your playing.  If you know a more experienced player, it can be a good idea to get them to try the instrument for a second opinion and compare the tone you both are forming.
Check the airway. Looking from the end of the whistle you blow into, look down the airway to check there in't anything in there.  This could be something which has become lodged/stuck in the airway, e.g. a fluff ball, or grime that has built up over time if not played for a while.  There should be no obvious obstructions in the airway.  If there is, a tooth pick should help.
Clean the airway As well as obvious obstructions the airway does need cleaned every so often.  It can be an awkward place to clean, and so some people put it off!  The airway is possible to clean with folded newspaper, or the tapered felt-pulls throughs which come with our cleaning kits.
Check the fipple The position of the edge, or labium, over which the air passes affects some aspects of the tone formed.  If this edge has moved slightly, then the lower register will sound very powerfully and the upper notes will not form at all - the effect should be very obvious.  If it has slipped a lot or become loose this should be visually obvious.  This would be a good reason to send the instrument back for repair.
Check the tuning slide The tuning slide should move freely but not feel loose.  You can find instructions for how to tension brass tuning slides on our instrument care page.  It is also a good idea to use some tuning slide grease to make the seal and keep the tuning slide lubricated.  If you happen to notice any moisture coming from the tuning slide as you play, tuning slide grease will resolve this.
AIrnoise / blocking If you are having issue with clogging or blocking please contact us as there a number of things that we can try.  This includes noise which is intermittent e.g. air noise that appears sometimes but others - like when the instrument is warming up for example.
If you have checked these things above, then please make a quick recording of you playing when the issue you are having demonstrates itself.     This doesn't have to be high quality - using a smartphone is ideal.  Email this recording to info@mkwhistles.com with a note of how long you have been playing and a description of the issue you are having.