Tie it tighter??? (um, I guess that comment didn't really help!) Here's my 3 solutions:
Try using less "slippery" yarn.
Try using LESS wool - for a more sparse pom pom. Then the tie-up yarn will be tighter!
OR conversely, use MORE wool for a VERY DENSE pom pom, and the strands will press against each other. Then after you trim it, you can't even single out any strands to pull.