Some of mine does that too. I think it might be to soft so when you pull it to tie the knot it strecthes too much and looks like that. I switched to the rolls instead of bolts and it seems to be way better now. Also maybe your cutting the strips to thin? I've gotten some of my tulle from Joann's and it didn't do it to me whe I cut it in at lest 6 in strips. Hope this helps. Btw I do like your tutus the way your doing them now
. They look fun and fluffy.