Re: heba diffuser
I had the same problem, the large one was too big and the small one was too tight.
You have to slide your stem in the circle in the center of the rubber part. If it doesn't fit, make small 4 small cuts (or as desired) in the circle you're supposed to put the stem in. Note don't make them too big and "work your way up through", it should still be extremly tight or the rubber will loosen and fall in winter when it's temperature changes.