Hi Lady P
I have this problem quite often as about half the RN4081 mouthpieces have to be jammed on to work. My answer is to keep the good ones (that fit easily on the atomiser). Just pick the cart out, toss it into your squeezy pile and keep the mouthpiece. Then when you're fitting a new one, if it jams halfway down the atomiser, don't force it, just swap the cart into your previous good 'un. If you do mistakenly jam a crap one on, dry hands, molars and twisting action works for me (as in so many areas of life).
Emp