My advice would be try to use a smaller baiting up needle, maybe your making to big a hole in the boilie. If that dont work use a bit of putty maybe just to secure it in. However before casting find a shallow margin a check the snowman is working correctly and the pop up isent to strong.
In my opinon use a 20mm bottom bait and a 15mm pop up, that seems to work perfectly, all depends on how boyount the pop up saying that.
Good luck and Tight Lines.
Ryan