why wont my muffins rise?

[Trikken]

I was given a muffin tray for christmas so was hoping to be able to make professional-looking muffins, but so far they've just gone flat even with longer cooking time. I've tried several recipes all having the same effect. does anyone have a suggestion to where i am going wrong? please help me from dooming more muffins!

ooh thanks for the 'asap' tip, that may be where i've gone wrong. I am a good girl and dont peek tho. will try being quicker and see what happens.

Completely late to this one, but just to add that when you mix muffin mix, it needs to be till the ingredients are just barely combined - ever mixing will stop it rising, no idea why! literally roughly fold in dry ingredients, scrape down the sides of the bowl, fold a bit more till no more lumps of flour, then bung into oven.

Make sure your flour isn't old.