Meringue needs a really clean bowl and make sure you don't get any shell, even the dust of the shell in the egg whites.  Whether the TM or beaters are used, I wipe the bowl and blades/beater/paddle out with either lemon juice or vinegar.  This cuts through any grease of any kind that may hinder the egg white whipping up either to soft or stiff peak.  The cream of tartar is an egg white stabilizer and is used with the egg whites in the whipping up stage, can also add a pinch of salt which helps to give a very morish flavour.
Interestingly my first attempt was a failure in the TM, due to grease in the bowl and on the paddle from using butter, even though I washed the bowl in very hot soapy water and thought it was clean, it wasn't now I make sure by using the vinegar rinse. you guys probably know all this, but just incase.

Hi Trudy
Went searching for Orange cake and found your orange muffins. I'm not sure how many dozen I've made, they seem to go as fast as I can make them. Thanks so much for this recipe its delicious.  I used an old icecream scoop with the flicky thing to scoop the mixture out, which gives a regular size and neat shape. Just intime for the orange and lemon season.